Most of the people who post on this board are from Western countries and are probably going to answer from a different cultural perspective than you have.

Having said that, I think that it was very bad of your friend to tell you these things (if he did it after you started liking the girl.) The only reason I can think of that it would be okay for him to say things about the girl is if he thought she was a bad person and did not want you, his friend, to be with her.

When you say "10 months" I have no idea how many encounters you are talking about.

Are you able to tell if it bothers you more that the girl was with your friend or if it was because of what she did? Do you worry that she has more sexual experience than you do? Or is it all mixed together?

From my point of view, the slight difference in your sexual experience isn't important.

I would guess that this girl probably wanted to date your friend but that it didn't happen. That is only a guess on my part, though, and I wouldn't recommend that you ask her about it. It would probably make her feel bad. If you keep seeing her, she may tell you on her own or she may not.

You sound like a principled person and like you are trying to be fair. I think that is very good.