At this stage, only the seller can assist him; paying through a third party is discouraged because not all vendors accept it for security reasons.If the vendor is not interested, his best option is to return the money, but not everyone has such a heart. If the transaction is disputed, the p2p mediator is likely to cancel the trade and suspend your brother's account. Sending or receiving payments in a third-party account is prohibited in P2P marketplace.
If the money is a stolen funds, the seller is already leaked to the crime - your brother broke the rules and should deal with it.
Another mistake he made was the fact that he didn't tell/agree with the seller on the account name from which he would be using to send the money, in the chat room which though is still against the exchange p2p policy but at least the exchange services would have seen the agreement when dispute was raised and probably released the coin