Who was the man God was talking to in Job

To answer your title question, that was Satan talking to God in the Job passage.

Satan was indeed cast out of Heaven.

Luke 10:18

And he said unto them, I beheld Satan as lightning fall from heaven.

To answer the next part of your question, which I will paraphrase as "How could this conversation between Satan and God take place since Satan was cast out of heaven?"

I see no where in the text that claims the conversation took place in heaven. As Satan was cast out of heaven it could not have been heaven. God can locate himself anywhere, and clearly Satan can be in the presence of God as exemplified by Satan and Jesus present together when Satan tempted Him,

Matthew 4:10

Then saith Jesus unto him, Get thee hence, Satan: for it is written, Thou shalt worship the Lord thy God, and him only shalt thou serve.

Albeit some do not believe Jesus is God, I see no text indicating Satan can not be in the presence of God. To drive that home, God is Omnipresent,

Jeremiah 23:24 King James Version

24 Can any hide himself in secret places that I shall not see him? saith the Lord. Do not I fill heaven and earth? saith the Lord.

We are all, including Satan, continually in the presence of God.

The next part of your question is, Why would God allow this suggestion? While it is not possible to guess the motives of God Himself, it is clear God allows Satan to tempt humans. Again the example is Satan tempting Jesus, and in Revelations, God will release Satan after 1000 years of bondage, full knowing he will deceive persons.

Revelation 20:7-8 King James Version

7 And when the thousand years are expired, Satan shall be loosed out of his prison, 8 And shall go out to deceive the nations which are in the four quarters of the earth, Gog, and Magog, to gather them together to battle: the number of whom is as the sand of the sea.

God allows Satan to tempt humans and it serves his Will to do so.

The concept of Satan as having been cast down from heaven at the dawn of history (or even before this) evolved over time. In fact the word "satan" appears only 14 times in the OT, and all but three of these verses are in the Book of Job. In this book it is not even a proper name; but "ha-satan" meaning "the adversary" or accuser. In these verses, Satan is not portrayed as God's enemy yet, but as the accuser and adversary of humans, particularly Job.

In terms of dates of authorship, Satan appears in the OT only after the Jews reached Babylon and encountered the dualistic philosophy of Zoroastrianism. Later, Jewish writers and especially Christian writers equated him with the "ancient serpent" in the Garden of Eden (Rev. 12:9). In addition, the prophecy of Isaiah 14:12 “How you are fallen from heaven, O Day Star, son of Dawn!" - originally invoked against the very human king of Babylon - was interpreted to refer to a previously unknown archangel, now called Lucifer, after the Greek word for the Day Star.

All of this is not to say that it is mistaken to think of Satan as an archangel who was banished from the heavens. However, in the Book of Job, he has not yet taken than form.

This article has an interesting view:

https://enduringword.com/when-was-satan-cast-out-of-heaven-live-qa-for-march-31-2022/

The author claims that Satan will fall four times:

• From glorified to profane (Ezekiel 28:14-16).
• From having access to heaven to restriction to the earth (Revelation 12:9).
• From the earth to bondage in the bottomless pit for 1,000 years (Revelation 20:1-3).
• From the pit to the lake of fire (Revelation 20:10).

According to that interpretation, the Job passage speaks of a time when Satan, though he had fallen from glory, still had access to heaven. That access may already have been rescinded or will soon be rescinded, depending on which timeline and theory of eschatology you subscribe to.

With that view, the person that conversed with God in Job 1 & 2 is Satan and not some other being.

As for why God permitted this conversation and granted Satan authority to torment Job, we do not know because Scripture does not tell us. Scripture does show us some of the outcomes of this testing. Presumably if Job's trials had a beneficial outcome it was intentional on God's part.

Some of those outcomes:

• Job pleaded that his words would be written down. They were and became not just any part of the Bible but likely the first book in the Bible written. Job's prayer literally created the Bible. (Job 19:23-27)
• Job cried out for a savior. Because Job was righteous, wise, and his needs were comprehensive and his situation grave, Job was able to describe in detail all the things that a savior would have to do and to be to rescue him. He literally created a job description for a savior. (See my book Job Rises: Thirteen Keys to a Resilient Life for a detailed analysis of this set of requirements. They describe Jesus' life and ministry including his dual nature as divine and human.)
• There are things we learn only through suffering. God's vindication of Job's words endorses the things that Job learned and spoke, enabling us to benefit from the man's travails.
• Job treasured words from God's mouth even more than food (23:12) or even his own life (13:15). This faith created the situation where God could speak massive amounts of divine truth into the world, because only one who treasures such words will write them down and fight to preserve them. God's speech to Job is the longest single speech by God in the Bible to any person.

The above are just a few of the benefits to the world that sprang from Job's trials. As a father with three daughters, I must add one more. Before his suffering, Job's first set of daughters would visit their brothers' houses for parties. The implication is that they did not have houses of their own. They were just women. After his suffering, Job divides his inheritance equally among his new sons and daughters. In fact, we don't even learn the names of his sons - just his daughters! Job was the world's first feminist. The rest of us are still catching up with this righteous man.