To Tell or Not To Tell

Here's the situation ... you are out on a date and you see your friends mate out with another member of the opposite sex. You speak to assure the mate and date sees you so that there is NO confusion as to rather it was them you saw or not. Do you tell your friend or keep it to yourself?

I have been in this predicament and I will tell you, after that situation I would keep it to myself.

How about YES they are still together 18 plus years later but I feel she and I are not close anymore as a result of this. Of course he denied it was him so there I was looking like a fool when she chose to stay.

I felt like I ruined a great friendship looking out for my friends feelings and ended up getting mine hurt in the process. So from here on out I will be selfish and keep my feelings from being hurt, salvaging my friendship and let him/her know if you see my mate out PLEASE tell me because I will act on it! Now, again I ask ...