Replying to My ex and my friend

alright so my ex and a friend of mine have been hooking up lately. They hooked up before when she and I first broke up and I found out and they both knew it pissed me off. Well that was a long time ago and now I find out that they have been at it again a few times (I hadn't spoken to her in two months). He and I are really good friends and he lied about it. He also lied about how often they would hang out. I knew they were still friends but he made it seem like they hadnt been seeing each other that often. She told me a couple of days ago and I was livid. I haven't spoken to my friend since, and I didn't think he knew that I knew. Well he texted me saying we need to talk and shit. Who do I be mad at here? I can't figure out if I should still talk to either of them. Advice?