<human>: Can you succinctly summarize the text in 1-3 sentences?

So, my best friend met a girl in summer who he eventually started dating after around 2 weeks or so of being together. However, he had to move away to another country halfway across the world at the end of summer, and after trying long distance for a while, they eventually decided it wasn't working and broke up. Recently I have been talking with her and things began to get more sexually related. Now after talking and flirting for a while she has told me she wants me to have sex with her, in which I would be losing my virginity and the fact that she is my best friends ex, makes me feel really guilty about going ahead with it (also I'm from the UK so it's all legal and what not). She is completely serious about it and has talked me through a few things I may need to know when it comes to doing it and having sex and we have already met up once and made out etc however we are not dating/seeing each other. Honestly, I feel like I should really tell him about it because we have been friends for over 10 years now, but she doesn't want me to tell anyone for fear of how people will think of her. 

I'm not sure how my friend will react if I tell him since he has another girlfriend now, but I feel like he wouldn't trust me if he found out about what was going on especially since they would probably still be dating if he was here. I'm not sure whether to tell him and possibly break his trust, or just not go ahead with it and tell her I'm not comfortable with doing it and stay as a virgin. I also think that if I didn't tell him I would feel really guilty afterwards, and even worse if I didn't tell him and he found out. So honestly I'm not sure how I should deal with the situation. What should I do r/relationships?
<bot>: Best friend of 10 years got a girlfriend, moved away and they broke up. She wants to me have sex with her and he doesn't know, and I feel guilty despite the fact he now has another girlfriend.