You friend’s account itself is not banned, but their internet connection was automatically blocked by Sentinel, preventing them from joining our server on any account.
Connection blocks are automatically applied when a user is banned from the server, so it’s likely a different account was banned on their internet connection in the past.
Unfortunately, we can’t do anything about automatically applied blocked connections. They are usually temporary, so your friend will need to wait until it expires. As we can not share the duration of the block, I suggest your friend to try log in every few days to check if the block has expired yet.