Why is this even a question? Of course you would tell your friend! You might try and suggest it to them subtly so they do the digging and find out for themselves but you are not going to let it lie. These things always come out and then you are the person that knew but never said anything.

A better question would be - Two of your friends are dating and one is cheating. They have asked you not to say anything to the other because it didn't mean anything and they are going to end it. What do you do then?

Tell them if it ever happens again you are telling, no excuses. Everyone gets one big screw up.