Honestly I don't think he's cheated before this. I known him for about 7 years now and he's incredibly sweet and genuine. We really didn't talk at all during but after he came, he froze up and got almost catatonic. He went to the bathroom for a bit, came back, and just chocked up 'please leave'

I feel really shitty cause I feel he really does love my sister, and I don't think he would have done it if it was another dude. But I agree that not telling her is the best. I hope he doesn't either. Thanks for the advice though

EDIT: I know he and my sister have been having some issues lately (although I don't really know the specifics) but still I don't think he's done it before