I wouldn't worry about embarassing him, I think that is your smallest problem. The main problem is wether to tell him or not. If you honestly think she loves him for the right reasons and they are both happy, and she stopped doing it before they met, I would just keep out of it. If she is still doing it or is a bitch I would definately tell him. What happened before they met isn't really relevant I dont think. I'm sure he has his secrets too.
