It is a really unfortunate situation, most probably he tried to extract the fork coins using the private keys keeping the bitcoin in the wallet and that is the biggest mistake he did, we have discussed about extracting the new coins and how to do it safely, the risk of using your private key with the coins in it is the most damaging thing you can do. I am not even sure who is behind BTC Diamond and BTC Private and i have not extracted anything till now, the only coin i moved is BCH and that too first moving my bitcoin to another wallet and that is the safest way.