You could just straight away tag him, that's not someone here could stop you from anyways.

I agree he did a bad move by using others work, it would surely be considered as very shady activity in real animation industry. But on the other side there should always be a way to correct the mistake or atleast do better work next time and most of the people here would agree he is pretty trusted with large amount of funds and would probably not repeat the same mistake again.

Rest is just your move, which would cost in some ways destroying a hard built account.