His account is proxybanned which means that he is accessing the forum from an IP address that is considered evil (that IP address was used to do evil things before like DoS attacks, spamming, having banned accounts, etc.). Such IPs are usually Tor exit nodes or proxies.
The only way around it is to pay the fee.
But is a mistake, he did never post anythin on BTT before.
As long as your IP is blacklisted, you have to pay the fee. It is not a mistake, it happens if you use Tor exit nodes or known proxies too.
Ask your friend to register using his own house internet or pay the fee.