Ok, so your girlfriend broke up with you, and your ''good friend'' is getting with her? :/ Sorry. Here is what I would do.
Take a deep breath. Think about the situation, and then ask your heart. Do you want to hate her, hate him or neither? Do what YOU think is best Simon. If you want to be happy, make your own choices. I am not going to put me as you, because I might guide you the wrong by saying ''If I was you, hate him and her, and be a moody bitch for the rest of your life''. I won't say that, and I'm not even saying that. Be happy, and let things smooth over. I do. It's how my life goes. I recently had a fallout with two close mates. We didn't get along after that. (This was like last friday). But I spent 3 hours talking to one of my mates on MSN, explaining I was normal, and saying I am myself. I always will be. With my other mate, I had to explain the tings I did to him. I spent 6 whole days to get him to speak to me and for things to smooth over. (Ok this is going off-topic, I will stop). All I am saying Simon is, let your heart and soul do the thinking, not your brain.
I hope you do the right choice. I really hope you do.
Thanks,
Ryan.