Should I tell my friend or not?

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Hi everyone! I need your opinion because I really don't know what to do... A couple of months ago, my friends and I met this guy at a local disco. A few days later, one of my friends started seeing him often and they now have a relationship, even though they're not going steady. Last saturday we went to that same disco and he was there with some of his friends. Later that night he asked me to dance and I accepted because it was not unusual because we had danced together on previous occasions. Then, when my friend wasn't looking, he started to flirt with me and asked me to meet him afterwords, after I took my friends home. I said I wouldn't do that and he asked me again. Again I said no and tried to explain to him that I would never cheat on my friend. But he didn't give up. He even called me the next day but I didn't answer his call. I decided not to tell my friend and keep this to myself. But if he starts doing this again, I'm going to talk to her and tell her what's going on. What do you think? What would you do if you were in a similar situation? Please help... I still feel like I'm cheating on her by not telling her this...