This.
There are boosters from certain countries who RMT the gold they receive to pay their bills. They operate like a Cartel, if someone is undercutting then considerably they will mass report them until they get banned. Reason being is for them, it is their livelihood not a game.
If your friend truly didn’t do anything wrong, this is likely the answer why he got banned. Best to write a detailed appeal saying what he was doing, the messages he received and why you believe this has led to a ban.
Fair warning though, this was fairly common practice for boosting cartels on Firemaw ERA and I don’t remember any undercut booster ever getting unbanned