now that you have told him, good. now your rep. is secure when he finds out the truth, then he cant be mad at you, you warned him. if you are a true friend, keep telling him so he doesnt find out the hard way; maybe even consider talking to the girl, who knows, it could just be a misunderstanding.. -theComing Inc.
Pruple, he doesn't hate you, my dear. He hates WHAT you told him and that now he has to face the fact, whether he tells you this or not, that he can't trust her now. It'll happen again and he'll find out because he's going to be watching for it. He'll get over the thing between him and you when the time comes...and it will come. So, try your best just to step back, try to see what he's going through emotionally and just be there for him when everything falls apart.
He would have been even more mad if you didn't and then he found out. You have nothing to feel bad about. You did the right thing. It's always a chance you take. She'll probably do it again. You have to decide if you still want him as a friend if he comes to his senses. You can hold your head high.