I got a secret, I got a secret...

I've been caught in that position, as I'm sure most people have, of having information that is important to a friend's relationship, but you're stuck with the question of whether it's a good idea to tell or hold your peace. On one hand, I clearly saw my friend's girlfriend out with another guy, but without knowing any details, I spilled the beans to my friend and ultimately lost a friend. On the other hand, If he would have found out from another source and also found out that I knew about the situation, then I would have lost him anyway. Catch-22!!! So, should you keep it to yourself and let the poor sap put 2 and 2 together and hope he makes 4 or do you tell with the understanding that your friend may not like what they hear? Spell it out for me...