I would simply turn around and leave. Change the locks, throw his stuff on the curb. Why even bother dealing with that sort of thing? I'd be happier with just a clean break than a messy argument about it.
For starters I would beat the crap out of both of them (him and the girl he was cheating on me with) then I would go home and destroy everything he has ever given me, put it in a box and put it on his front door step.
I've only had one boyfriend to cheat on me, but I didn't see it. I was told by the owner of the skate park that he was there the night before and he was all over some girl named "Natalie". And the owner of the place said that they had to kick him out and then he went and urinated in the owner's car so they banned him from coming back (which explained why he didn't feel like going skating). So I called him and confronted him about it and he remained silent, so I asked him again, "Did you cheat on me last night?!" and then all he had to say was "Maybe we should take a break" and I hung up. Then he started texting me asking if I was mad at him!! And the only reason I didn't beat him and her was because 1: I didn't even know her or knew what she looked like (lucky her) and 2: my parents told me that I couldn't see him anymore, because they said I deserved better. So they both got lucky, cause if I had seen them I would've seriously hurt them.
Well, I have been cheated on before. I never actually walked on them but my friend told me and took pictures of my girlfriend cheating on me (kissing a guy on a restaurant). I went home and I sent her and email that I knew she was cheating on me and that this was her loss, she tried to call me and tried to send me emails, but I never answered back. a week later I had another girlfriend which I took on a vacation to New York. I knew my ex was checking on my Facebook status so I posted a ton of pictures of me and my new girl all over new York kissing and holding hands. I just made it look like it didn't even faced me that she cheated on me, more like I was glad that she did because now I could date whoever I wanted. 2 years later she sent me an email saying how she knows that she had a good thing going on with me and she messed it up. She says that she regrets what she did and that she hopes that at least we could be friends. I never answered back to her. I don't need friends like that. I have plenty of friends so is not like I am desperate for new ones.