Brief info: I am 15, 16 next year, and most people say I'm mature for my age, so I've decided to take that on board and be mature by asking for advice rather than doing anything irrational. Here's the story;

When my ex broke up with me completely, I'll admit, I cried a little because I really did like him but I wasn't all that hurt to begin with. But about a week just passed, and well, I've only just found out that he kind of cheated on me with my "best friend". The reason I've said kind of was because, they didn't actually have sex or anything like that, but they were doing all kinds of things over the internet. The problem is I see her every day at school and now that I know, I don't know how to act around her. I've still been talking to her and being friendly, although I have tried to avoid conversation about my ex with her. She's still pleasant to me because she doesn't know I know. I'd just like an opinion or two of whether I should act any differently, confront her, or just let it go and try not to be hurt by it. Every opinion counts :)
Thanks.

I would just let it go. You said you didn't care that he broke up with you. You could tell you that you know about the two of them and t's cool with you.

Thanks.

What makes you think she won't go do it again? I wouldn't confront her or make a huge scene out of it, but I sure as hell wouldn't be her friend anymore. That's just my opinion though.

I agree with Cobra. Even though you broke up and are moving forward, you still have a relationship with your best friend. This will always be between you two unless you get it out in the open.

I have a slightly different opinion - the definition of a best friend is that you can tell each other anything. I would just ask, gently. In private, not at school, but in person. You can say 'even though it's all over I feel sort of hurt by the secrecy.' And I think she might not be my friend after that, unless you have a sense that she's telling the truth. I might look for another best friend regardless.

I would tell her that you know. That is not a thing a best friend would do. Let her know that you know and that you don't think it very nice.
Do you know for sure something went on between them?

I would let it go if it's not hurting you, but if it was I would ask. POLITELY.Because what you don't need is a fight, no no no fights. You're in a tough situation, everyone can see that who has seen this question. But... honestly
If I was you, my temper would have gone off, so I am proud of you, VERY PROUD! For keeping your cool. Honestly if he was your boyfriend (now ex), and she is your BEST friend, should wouldn't of done that. Best friends fight the temptation for their friends.

Thank you all very much for your help :) I do know for sure that something was going on between them :/ it is still upsetting but your opinions and advice are giving me a better outlook on it. Thanks again.