What Kit Kat should really do is talk about all this with her friend, the guy's girlfriend.
You don't have to be cruel with her, you just had to tell her the truth. Maybe you are thinking you're protecting her by not telling. Maybe you're afraid to lose her as a friend. But if you are really her friend, why did you almost shag her boyfriend?
This situation is tragic. There's a girl whose boyfriend is a scumbag and is trying to have sex with other girls (are you sure you're the only one?) but doesn't leave her because it's convenient to have a stable girlfriend while you're fooling around others. Unless she's fine with this and is doing more or less the same (which for adults is a perfectly viable options if the 2 agrees about it, but I bet she doesn't know anything about his "hobby"), she's going to have a terrible wake up when finding out. Now do you want her to find it out now that perhaps he's done that only with one of her friends, or do you want her to find it out later when he scored with many and the whole town already knows?
The guy is just on a bad track. If he gets away with this, he'll probably do the same all his life. I think you might be doing him a favor too if you expose him.