my friend's account is missing

I had a friend on the lordaeron wotlk, and for whatever reason I can no longer see his account. You know how you can see someone's account in your friendslist when they are offline? Well, I can't see his account anymore. His character name was "garetch", and he had another character on that realm named "galachad". There is no trace of either of those characters.

I also received a letter from him, that he told me in advance that he would send me. When I went to check my mailbox today, the letter was definitely from him because it had a robe attached that he said he would send me, but it listed the sender as "unknown".

Do you guys think that my friend got his account banned?

That does not happen when you get banned. Most likely he removed his characters.

Deleted or faction changed, probably deleted since he sent you some stuff.

If you change factions people can still see your name on mails, crafted gear/items, etc.