Should You Tell Your Friend?

Let's say you're out with a few co-workers or acquaintances at the movies or out to eat and you spot your best friend's significant other with someone else. Or what if your car has put you down and you desperately need a ride home, your friend sends his/her S.O. to pick you up and they hit on you. In either of those situations should you tell your friend or let them find out on their own? Is it ever okay to tell your friend that you suspect their significant other of cheating on them? How do you know? I personally have been in this situation before and my advice would be to stay out of the situation. Unfortunately, in most cases, although the person SAYS they want you to be honest with them, the truth hurts and the situation can quickly turn into something you definitely didn't want it to turn into. Your friend may blame you for what happened if their significant other hit on you or get upset and think you're lying to break up their relationship if you tell them you've seen their S.O. with someone else. Now, there are ALWAYS exceptions to every rule and although things didn't go the way I felt they should have in my situation with my friend doesn't mean that would happen in your situation, but if you're ever out in that situation I'd certainly say tread lightly. It's a tough situation to be in and you never know how people will react. What are your thoughts on this?