Simple answer: none of this is as simple as your friend pretends it is.

For openers, belief is not static, it is part of the continuum called life. What I believe and accept today is different from what I believed and accepted a quarter of a century ago. (I was not a confirmed Catholic then). What someone believes or doubts today may, through a variety of influences, change.

Faith as actually practiced is as much experiential as it is requirements based for real people, as opposed to people depicted in a hypothesis or argument. Faith as a life journey can be quite difficult. Jesus warns his disciples of that in Matthew 16:24

(KJV)Then said Jesus unto his disciples, If any man will come after me, let him deny himself, and take up his cross, and follow me.

The cross is (among other things) a symbol of suffering for righteousness' sake. The allusion in the above passage includes the expectation that in following His Way, burdens and troubles will attend. One of those troubles is doubt.

Problem element 1

A Catholic that doesn't accept the assumption of Mary will be excommunicated from the Catholic church

If that protest/assertion is made publicly, then the believer is opposed to a dogma of the church. Will that necessarily result in excommunication? No, but depending on circumstances, it might move from an issue of doubt to one of heresy. As @AthanasiusofAlex so kindly reminded us,

A Catholic who publicly and obstinately denies the Assumption of Mary would incur a latae sententiae excommunication based on Can. 1364 §1, since that is a dogma that must be believed with divine and catholic faith. That does not mean that the person will be automatically condemned (a common misconception; after all, he may be acting in good faith, albeit mistakenly). Before receiving the sacraments again censure would have to be lifted.

This raises the distinction between heresy and doubt. From @AthanasiusofAlex again:

The key thing with the delict of heresy is obstinacy. If you affirm something materially heretical, but then retract it once the competent authority calls you out on it, you don’t incur the penalty. Also, when we say that heresy can be “obstinate doubt,” we don’t mean that the person says “I am not sure I believe that dogma;” we mean that he affirms (publicly) “that dogma cannot be affirmed or denied.” It is an objective, not a subjective, doubt.

If someone is excommunicated, and does not then follow up with penance and reconciliation in order to return to be in communion with the Church (however that works out, reconciliation is an intensely personal experience) they will remain outside of communion (the "ex" from the Latin root of excommunicate) and are at risk of damnation.

Beyond matters of censure, the clergy are charged with encouraging, teaching, and inspiring everyone in the faith community to embrace the belief, and to counter and overcome doubt through their teaching and other pastoral efforts. Most clergy are astute enough to realize that simply waving a sheet of paper with a dogma on it is not as effective as pastoral counseling and engaging with people. What a novel thought: treat someone like a person, not like someone to be talked at, nor as a two dimensional construct that your 'friend' presents. The priests whom I know, save one, are all very good at that pastoral counseling thing, and at engagement.

If you parse "ex communicate" as meaning something close to the original Latin -- outside of the {faith} community -- the person so doubting and objectively professing rejection of the dogma has removed him/herself from the faith community. Once one turns away from the Church, it is a short distance to turn away from God, and willful turning away from God is taught as the path to damnation.

CCC 613

To die in mortal sin without repenting and accepting God's merciful love means remaining separated from him for ever by our own free choice. This state of definitive self-exclusion from communion with God and the blessed is called "hell."

It takes a bit more than having difficulty with one element of belief to turn away from God, however. Doubts have plagued believers since the faith began. St Augustine and Mother Theresa are but two whose diaries reflect the challenges, the hard road of picking up that cross and following Jesus.

What anyone actually believes, in their heart of hearts, is only known to themselves. If someone does not believe or accept that dogma sincerely, but keeps that doubt to themselves, there are not grounds for censure, nor for excommunication -- the problem of heresy does not arise.

FWIW, one of our deacons cites as 'general knowledge' that only about half of practicing Catholics sincerely believe that the host/wine as presented during the Mass really is the body and blood of Jesus Christ. That 'well known' understanding about a matter far closer to the heart of the Catholic faith doesn't result in mass excommunications. Why? *The Church never gives up hope for a spiritual breakthrough and the Holy Spirit working through the faith community. Among other things, Jesus brings us hope, the greatest hope being that of eternal salvation through him, and with him.

Problem element 2

2. A catholic must believe in the assumption of Mary

This raises an interesting point about dogma. When by the authority of the Magesterium of the Church a dogma is declared, the leaders of the faith community draw a line in the sand. (It's not done very frequently). It may present a challenge to the faithful.

• What do you really believe?

• What do you willingly accept/embrace?

• Will you sacrifice your own pride/ego and take it on faith?

• Can you carry that cross? (In this case, the cross of doubt).

  How people respond to such a challenge, if their belief encounters doubts, will vary. Some will respond via prayer, some through learning, some via pastoral counseling, some by just ignoring it, some by leaving for other paths. (And various combinations of the above, and much more).

  What people truly believe can only be answered in that quiet place where only the person and (if the believer is open to him) Jesus exist ... and the truth shall set you free. (John 8:32)

What if you just can't?

If someone just can't believe it, or just won't, they face a number of possible choices:

• Possible growing discomfort with the Faith. Without the help of others among the faithful, and/or the pastor, that discomfort will often not be resolved. If nothing else, that cross of doubt gets heavier and harder to carry.

• Continuing to follow the faith, however imperfectly, and being open to a spiritual awakening or a new understanding. God is reasonably patient with the motely variety of sinners who follow the Word become Flesh.

• continue list for pages ... with each person the challenges are posed differently

Bottom Line

Your friend is arguing as though faith is something two dimensional. Faith is more complex than that. It's fundamental purpose is the spiritual relationship with God through Jesus Christ. (From the opening article of the Catechism of the Catholic Church.)

I. The life of man - to know and love God

1 God, infinitely perfect and blessed in himself, in a plan of sheer goodness freely created man to make him share in his own blessed life. For this reason, at every time and in every place, God draws close to man. He calls man to seek him, to know him, to love him with all his strength. He calls together all men, scattered and divided by sin, into the unity of his family, the Church. To accomplish this, when the fullness of time had come, God sent his Son as Redeemer and Saviour. In his Son and through him, he invites men to become, in the Holy Spirit, his adopted children and thus heirs of his blessed life.

What rules and dogmas can do -- for all that they depth and texture to the faith -- is aid and abet that journey. If the believer hangs in there with Jesus, and is faithful even when struggling with doubt, they'll meet him and he'll do as he has done since the beginning: forgive. (What's that he said on the Cross? "Forgive them Father, for they know not what they do." (Luke 23:34))

Coda

If a member of the faith community just can't accept that dogma, there are likely other things someone can't or won't accept. That can lead to a different form of self-excommunication, which results in trying to find another path. There's risk in that approach -- it is a choice that can result in point 2 becoming true even if it is not true by default.

Jesus used to chastise the Pharisees and Sadducees about their being so wrapped up in the letter of the law, while he tried to turn them toward the spirit of the law. (The Gospels are loaded with examples, pick one). You could do worse than to so engage (in a loving way) with your friend, using Jesus' example as a point of departure.