Quoting BαƚMαɳ:" Listen to your heart. Figure out if you love him or not. If not, leave. I personally don't think that ... [snip!] ... the cheating if you don't want to be with him. But if you plan on staying with him I would definitely be honest and upfront."
This. If you really have no hope for your marriage, and you don't love him, why hurt him and tell him?
I think it's pretty smurfed up that it's your FIL's best friend, though. I guess if you're going to continue the relationship, you're going to have to tell him about the cheating.
Don't continue a quiet affair. That's smurffy, disrespectful, immature, and just plain wrong. Be a big girl about it.