How to confront a Cheater that you already caught and still denies...

I caught my SO talking to a girl that he talked to before and I found some inappropriate things. I was told by both nothing happened. Fast forward I become friends with this girl, supposed to be best friends. With in the last week see them talking again. SO made his buddy mad and he snitched on him and I ended up getting an abundance of evidence! Asked SO about it again and he still denies. I haven't whipped out all the evidence yet because I want to hear it from him. Confronted the "best friend" and denial from her too, but she no longer wants to be my friend because I said I saw the evidence. I'm just so hurt and lost...