I think it depends entirely on the people involved. I have one friend who I know would want me to tell her, right away. I know this because she's said to me in the past, on many many occasions "You would look out for me, you would tell me the truth no matter how much it hurt, I can trust you to be honest, right?" With that friend, I would feel it was my duty to say something to her.

I have other friends whom I know would want me to stay out of their business. I know a couple where she cheated on him for quite awhile and I knew about it and didn't say anything to anyone. Then eventually she broke it off with the other guy and stayed with her boyfriend, and he never knew she cheated and still doesn't know. But in the end she chose to lose the other one and stay with her boyfriend and they are happy now, so in that case I'm glad I didn't say anything.

Of course at the time, I didn't know that was going to happen.

So try to think about your friend, and based on who they are what might be best. I really can't tell you what to do more than that.