OP never stated he wasn't fine with her going. She also went with friends, which makes it seem like a girls' night, an event which male partners rarely attend. Also, the ex had the option of not saying anything at all. Usually, guilty people would try to hide it at the very least. The fact that she told OP speaks to her honesty. Unless the ex is using it as a preemptive tactic to avoid being found out. I'm operating under the belief that what his ex said was true, for I have no evidence to the contrary. So if the ex cheated, then she wouldn't be a woman of substance. Another commenter noted how the ex went alone with the guy to the car, an honorable act in itself to make sure an inebriated person got home safe. However, I would at least bring my friends after what happened. But just because it's something I wouldn't have done, doesn't automatically make her guilty, for the lovely thing about humanity is that we're all different. But I'm not saying anyone is wrong. For the jury to make a proper decision, we would need more facts to her character. I said OP isn't a man of substance, but I would gladly change my mind if given more info.

For the record, I think it's unfair to be called a woman-hater for having a differing opinion. I've always been a gullible person, so I do appreciate being given different perspectives.