first, you've been with this guy for 4 years, so something is working. Yes, it's worth patching up. Definately do a lot of premarital counseling before getting married. Don't skip this step, you will regret skipping it. You are hurt as if it just happened, because it just happened to your heart. It's probably something in his past that he's regretting and completely wishing you never knew. I think I'd prefer it was only physical, it's just sex and that's it. If he had been in an emotional relationship, that would be much more difficult.
As far as telling her bf, leave that one alone. If he asks you, then tell him honestly. Let's say for example, you are all at the same party. You just happened to be chatting with her bf. You'll talk about lots of things and not bring up this topic. But, if it comes up, you can talk about it. Otherwise, leave it alone. I think you are just trying to hurt her the way you are hurting, and that won't benefit anyone. She needs to tell him, but she needs to do that in her time, not in hers. She should have told him a year ago when this happened. But, if she had done that, her relationship with him probably would have been over. The same is true that your bf should have told you, but he didn't because then your relationship would be over, and everyone would be hurt. They were stupid. Hopefully, everyone learned something.
Just be 100% sure, through counseling, that this is the right relationship before you say "I do"
Love 
