Just wait, bad things don't last forever. They will get caught and she will have hers in the end. Just let them be. I know it is easy to say, but the truth always has a way of coming out. Hang in there you will see.
May,
I have been in the same boat. Do I tell my ex's girlfriends husband she is cheating on him or not? My ex has told our children only half the truth. None of them know she is still married. The same with the rest of his family. All he has told them is that he is in love (yeah right, he does not know what real love is). It is all LIES and Cover ups when he talks to his family. There are days when I would REALLY like to expose him as the liar he is. This going on in a Catholic basis family.
I also believe she is just playing him.We have been divorced for 6 months and she gives no indication that she is going to leave her husband (who I hope lives to 100). She seems happy to just have an ongoing affair.
When I get really upset about this I just try to remember that they don't have a REAL relationship. They can not be there for each other without exposing their relationship. If one is hospitalized the other can't be there for them. If a family member dies, same thing. They can not spend holidays together.
I have always felt if you have to hide what you are doing you shouldn't be doing it. Someday it will come back to bite you. This is what they have to look forward to.