Go round there and be straight and say

"I didn't know how to react at first as I was shocked and hurt but I've been seeing (hisname) too, I'm sorry I didn't say anything sooner it's just I was taken back that we have both been seeing the same guy and I needed time to process what's going on. But I'm cutting him off as it makes me sick, he spewing all these lies to me and then coming having sex with you that is why i tried to tell you he was using you without actually saying why because i felt sick and used myself "

As you pull up to her apartment. Before you say this☝️

Text him and say..

"My friend (hername) just showed me a picture of the guy she is having casual sex with, and it's you!! She obviously didn't know about us, but now she does I don't want to hear any excuses or lies anymore.. what ever this was is now over and I will be promptly blocking you"

Then block him, exclusive or not he made it clear to you he only seeing you to keep you sweet and that's disgusting.