Please post a link to proof of the hack. And if possible the proof should include the ID. Because it is not very hard to call someone a cheater. In most cases good players will be accused.
I do not know about Crysis, but the Source engine allows to record while playing. And you can get the IDs of all connected players in this demo. Pretty much the best you can do to provide proof.
Screenshots can be manipulated, vids can be done with players who just changed their nickname.