Hey there! I can’t look into player accounts at all, so I have no context as to why your friend was banned or what was flagged to cause it. The best thing to have him do is open up a ticket with support, as they are the ones that process all bans and appeals, and unfortunately no one on the forums is able to process a ban appeal or investigate here.
Please also see the following post as well: Regarding RMT Penalties
As a reminder, trying to appeal or attain information on a suspension or ban within the forums is not effective, as forum staff does not have the tools required to look into player accounts, bans, or appeals. Moving forward, threads created with the goal of appealing a ban will be locked, and the player will be redirected to Customer Support to discuss or appeal.