If he really doesn't know, he can write in to chess.com and ask.
If he does know, he can write in, apologize, promise not to do it again and he will possibly be reinstated. (It would obviously depend on what he did.)
I have seen many people come back from the dead.
I met a player here at chess.com, he was a diamond premium member. Then suddenly his account was closed. When I talked to him personally, he does not know why his account was closed.
Commonly players get banned due to cheating. I doubt my friend is cheating. Maybe yes, maybe not. God only knows. Are there any other ways to get your account closed?
Oh, another way is to invite members to a group using messages. In other online games, sending a message to a prospective initiate is courtesy. In chess.com you get banned for it.
So I have two, any others? Being a premium member means nothing if you get your account closed.
Would the player receive a warning? Like "please refrain from doing 'something' else your account shall be closed". It seems not. So your account can get closed anytime?