If you knew that a friend's significant other was cheating on him or her, would you tell your friend the truth or keep it to yourself?
*sigh* I have been in this situation. And yes, I would tell my friend. Or, rather than telling them myself, I'd grill the cheater to tell them. I have no empathy whatsoever for one who is cheating; you reap what you sow. If the cheater was cheating in the first place, then s/he is definitely not worth of my friend's time, no matter what the reason. And if they don't even have guts to tell the truth to the person they were cheating on, they are not worth my time, either.
At least stand behind what you have said and done!