How much do I tell her?

My best friend has had a very complicated relationship with a guy for about 3 years. Although they've never done anything, they've always fancied each other but never at the same time. Generally it has been him liking her more than she likes him.

A few weeks ago though, they had a massive fight at a party. Until today I'd only heard her side of the story, which was basically that he continually insulted her for no reason except that she wouldn't kiss him. Since then they've not been friends.

She is now going out with someone else - there was very little build up to it and it's going quite fast. According to everyone else at school it's a completely random and stupid match - I've seen them together more than anyway else though and I can see that they do go quite well.

Today I went to a party which the boy who she had the fight with was at. He was a bit tipsy, and we ended up talking. He told me his side of the story - that it wasn't completely unprovoked and she shouted first. His story sounded pretty reasonable and now - although I so want to completely believe in my best friend - I can't help thinking that her view of events is too extreme and slightly unfair.
He then went on to say that he still loves her (I say love rather than fancy because it does seem to be much more than a crush) and when he found out she was going out with this other guy it felt like someone was continously ripping up his heart for two days. He said that she is being stupid in her relationship and is just kidding herself.

Of course this was all "in confidence" but I feel like I really should tell her. I never realised and I doubt she did that his feelings were so deep and strong - he started crying during this conversation (maybe just at the emotional stage of drunkeness, but he was talking pretty seriously).

I don't really know if telling her any of this would be helpful at all, but it seems like she should know. And if so, how much?

Does anyone have any advice of what I should do? Another big part of the problem is that he is totally convinced that they would work brilliantly as a couple - but I can see that in reality they never actually work, and things always just get complicated. She wouldn't brake up with her boyfriend for him, but I feel so bad about how down this guy is feeling at the moment, so that she should know and try to make an effort to be friends with him.

Or is that stupid and should I leave it?

I'm so stuck and I can't get his expression out of my head

Yeah, I guess. I mean the only reason I kind of want to say something is because she is still incredibly angry at him. I think she is using her relationship to get back at him a bit too, but I don't think she sees how much he's already hurting. I want her to be nice to him But yeah, he is very wrong in thinking that they are ever going to be in a stable relationship.