Should I Stay or Should I Go?

Selfish, wrong, I don't know. It's not for me to judge.

I do not think you should tell him. It's not certain that he would be grateful for this knowledge as you were. This knowledge has a lot more potential for disaster by being told.

As for your boyfriend, if you value your relationship at all, and you say you were headed towards marriage, cheating is never excusable or understandable. If I found out I was cheated on I would leave without further questions. Your man lied to you for quite some time and you had to find out through someone else. A dog is a dog whether in college or not.

SusieG, welcome!  And I'm sorry you're grappling with such questions

Here is my assessment on the 2 relationships/ situations, but it's just my personal opinion.

1) DO NOT tell the boyfriend of the girl.  You may feel that you are doing this out of being a good samaritan, but it is in the past (over a year ago) and not your business.  If she chooses to tell him, then he will.  Maybe she wishes she never did this.. maybe it was a moment of weakness, maybe she was having doubts about her bf and wanted to see what else was out there.  Maybe she was lonely but swears never to do it again.  Maybe they have an open relationship.  But that "mysterious" call can cause nothing but trouble.

2) Why are you writing about this now if it was over a year ago... did your bf just tell you?  If he did kudos to him, the guilt was really getting to him and he really will never do it again.  if it was someone else... well either way Forgive him but Don't forgive him easily, make him earn your trust and respect.  But don't throw away 4 years if he is perfect in every other way over a small affair.  Unlike undercovercrab's cut and dry opinion, i think ppl make mistakes and should get another chance.  Of course if he makes a habit of this...

"I found out because a friend heard thru the grapevine, gathered the facts and told me."

He didn't tell her, she found out. And they had sex 3-4 times to the OP's knowledge. Read her post. It wasn't a one-time mistake.