Cheating

So I’m in high school, and one of my friends has a boyfriend. I start casual conversations with this guy because he seems nice. He then starts to say things that nobody with a girlfriend should say to another girl. The conversation kinda went like this:

IRRELEVANT SUBJECT:

Him: “I would say something of yours is big, but you’d tell everyone”

Me:”wdym? I won’t”

Him: “you have a really big ass”

Me: “Uhm disagreed...”

Him: “Prove it”

Me: “wdym, Prove it?”

Him: “prove you have a small ass then” (how else are you supposed to prove you have a small ass other than ass pics?

I kinda forced the conversation to a close then but now I’m wondering how I tell my friend that her boyfriend just said those things to me?

*If any of you are mad about me talking to my friends boyfriend, stop. It was just casual conversation and I said ABSOLUTELY nothing for him to say those things. I also started this conversation by just asking if he was wearing mascara, something completely innocent and laughable. She’s also not a best friend, she just a friend, but I’ve known her for years and I feel like she has a right to know. I also came on here for actual advice so please give actual advice :)

Any help?