I Don't Know What To Do?

So there's this guy that goes to my school. Let's call him X. So my best friend who had a boyfriend but broke up with him because she began to have feelings for X. She really liked him and told X she liked him. They started to talk but I could tell he wasn't really into her. A couple weeks pass and she stops talking to X and gets back together with her boyfriend. Mind you they've been together for 10 months now. She told me she still has some type of feelings for him but she wants to get over him and focus on her current boyfriend only. So she said a couple times to her friends and myself to date him so she'll get over him. I think she was being serious but I played with her saying, "okay I'll date him". Then today I get a text from X saying he's really into me he thinks I'm beautiful, and he doesn't want this to get in between our friendship, but he really likes me.  Now he asked me how I feel but I don't know what to say because I don't want to hurt my best friend nor our friendship but I don't want to hurt him. I've always felt something for him for the longest and it really hurt me when my best friend liked him because I've liked him for a while. What do you think I should do?