I thought about this and i actually don't agree with the whole asking the cheating partner to own up before you have to step in...I thought i did but it's technically blackmail and frankly it lets the cheating partner off. If they won't own up then i don't see how forcing them is going to help - they've already had that opportunity and it might allow them to lie or manipulate the situation into something more advantageous for themselves. Better i think to go to the Friend direct, express concern that it's none of your business and that by coming to them direct you're already involving yourself as much as you'd want too in the matter. At the end of the day your interest is in them, not their partner or their relationship - them.