Just wanted to start by thanking everyone for their responses and sorry I haven't responded to everyone but its a rough week.
I spoke to my girlfriend and she confirmed that they were sleeping together and she did cheat on her ex with him. She said it wasn't a good period of her life and things weren't great in her relationship but thats no excuse to do what she did. She never told anybody because she was embarrassed and then when the prospect of us being together happened she was afraid I would reject her if I knew.
The worst part is that she admitted that about 5 or 6 months into our relationship after a night out my friend starting messaging her and things got inappropriate. By inappropriate she meant that my friend tried to get her to invite him over. She said no but they did talk about their time together which was definitely fucked up. The next morning she messaged him and told him not to contact her again and there has been nothing since. She said she's deleted everything but she can probably recover the messages to prove she's telling the truth but that some of the stuff will be hurtful for me to read. I don't know if I want to read that.
So thats it, my friend is a complete piece of shit and Il be having nothing more to do with him. I don't know what I'm going to do about her. Our families are very close and are both devastated as well. The whole thing is a fucking mess.
TLDR: My girlfriend confirmed what her ex told me about her and my friend.