The form is here: https://www.uscis.gov/i-485 However you can't just file it and expect approval. You have to have the proper status or already be admitted on an immigrant (or dual intent) visa. He almost certainly has none of these things. He should sit down with an immigration attorney and his specific details to see what options he might have.
It's going to run over $1200 to apply and he's just throwing his money away given what you've said at this point.

