Guy ramble; feel free to ignore

Well, I found out that guy, my 'guy best friend', is sleeping with one of my close friends.

She's his ex, it shouldn't upset me. Honestly the only reason I've never told her about me and him having a thing was because I knew I was in the wrong for going there with her ex but he said he loved me before he started seeing her.
She told me yesterday when she was drunk. I didn't tell her that I'd been meeting him and kissing him around the same time. She did say she knew they weren't exclusive or anything and that obviously they could both get with other people, but I don't think she quite meant he could get with her friend.
Now I feel sick.
All the times I've gone to his, all the times I've met him for walks or drives.
Did they have sex in his car, the same car that we've kissed in? The same car that he's tried to go further with me in.
And at his uncle's house, where we just lay on the sofa watching films, nothing happened there between us, not really, but did he fuck her on that sofa too? He had the keys to that house for a week, I only went once.

I knew he was sleeping with someone. He told me he was. He would always text me to invite me round to try and convince me to do stuff, and when I said no then he admitted once he texted another girl, a girl more willing. He knows I'm technically a virgin, so he knows not to push me too far.
But I feel sick now. I knew he had two main people he was trying with, two people who he fancied, me and one other. Now I get why he wouldn't tell me who the other was. He said I didn't know her but that he wasn't going to be a dick and tell me who she was, which I accepted because I didn't want him telling people about me either.
But my friend. He's sleeping with my friend.
He broke up with her last year, he told her he liked someone else and didn't feel right being with her.
He told me the person he liked was me.

It would break her heart if she knew. I thought they were finished, I didn't think they'd spoken since they broke up.
I don't know what to do. I texted him when I heard and told him I needed to talk to him.
She says she's got a free house soon and she wants to invite me, a couple of other girls, him and all his friends round for a party. Her plan was that she could be with him, then kind of leave me and the other girls with the pick of his friends (his friends are all hot). But I don't think I want to see them together.
I didn't want to hear about it. Whenever I'm drunk with him we kiss. It's just what we do. And I know him, even at a party at her house with her there I would bet money that he'll still come onto me. Maybe in the hallway or away from everyone else, maybe in the main room, but I know him well enough to know if we're all drunk together then something will go down.

I don't even know what to think.
Sorry this is such a ramble.

On the plus side, my mum bought loads of vegan food today. Lots of tofu and lentals and stuff. I really hope this will work for me.