What would you do?

She would just have to get mad and if that is what she did then I would figure that she wasn't really a "good" friend in the first place. A good friend would have realized that you were not trying to hurt them and would appreciate the fact that you were looking out for them.

that kind of news is really hard to hear, even if your really close to your friend she may get upset because she's just hurt and sometimes when people are hurt they get mad at the person bearing the bad news, im just not sure if i would get involved if i were in a situation like that becuse iv seen friendships break up over relationship issues before, sometimes trying to be a good friend can cost you a friend as well, theres is no easy way to handle a situation like that, i partly agree but the other part thinks it might be best to stay out of it.

what if your friend gets mad at you anyway just for telling her?

OK, but what if your friend gets mad at you for telling them iv seen it happen before, some times the person with the news gets all of the anger reversed on them, and you lose your friend anyway.

It has alot to do with the way you present it to your friend. If it was reversed you would want to know. Just make sure you know for a fact. It is better to know that you did the right thing, then to know your friend was being wronged and you did nothing about it. To me if you know they are being cheated on and you stay quiet then your not a friend at all.