Guy friend cheating on my friend who he is dating with me?

There is a group of friends who are in one of my classes who I hang out with. One of the guys in the group is dating my friend. I actually kind of have a thing for him becuase he is super funny and nice. We started texting a little and now just mainly snapchat and sometimes use the snapchat chat feature (I personally like using snapchat instead of text for everyone so this wasn't weird). Well he would always say how pretty I was or how he liked my boobs and I thought he was flirting, until he asked out my friend who is also in our class. I was pretty bummed but I moved on. We kept snapchatting and he kept getting more and more flirtier, saying how I am gorgeous or my body is super nice (I do send mirror pictures of me wearing like a sports bra and PJ pants and honestly my boobs are jsut really big so they are all over the place (I can't control it)) and always screenshots my snaps becuase "i look hot". He asked what my bra size was and I told him bc I dont hide it, if someone asks for my cup size i tell them and he isn't the first of my guy friends to know. Well then he asked for nudes. I told him no bc I dont wanna be a home wrecker and he is still dating my friend. He keeps saying he won't show anyone. I am upset becuase I know he is using me for my body and I still like him. Do I tell my friend who he is dating what he is doing? Me and her aren't close but she is really close with my BFF. I dont want her to get mad at me for flirting (because honestly I was and i feel so stupid and bad becuase I knew in the back of my head he was just using me for my body and he was still dating my friend but I had a major crush on him and I guess I wasn't thinking straight) and I dont want him to get mad at me for telling her. This is a super awkward place to be and I dont know what to do. HELP!
Tell my friend what he is doing eventhough we aren't close and I risk ruining my friendships with both of them and possibly more people
Vote A
Don't tell anyone but stop flirting
Vote B
Other (please explain in an answer)
Vote C
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Guy friend cheating on my friend who he is dating with me?
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