I tend to agree with the others - it's a really difficult situation to be in. I've been cheated on and I suspected (and was right to) that some of my friends knew about it at the time. I wished that one of them had been honest with me, as I felt like a complete fool when I found out.

It really isall dependent on how well you know both parties, and how you feel morally about it. I feel really sorry for the person being cheated on more than anything - they obviously have a right to know what's going on, but it's how comfy you feel about being piggy in the middle.

I think a quiet word in the shell-like of your cheating friend might be called for, even if just to let them know how uncomfortable you are with the situation and how you do not want to be party to it. If he/she really is your friend then they will respect that.