Long tale that I won't recount in full for fear of exposure. DH has found out FIL is a serial cheat, in a really big way. He's devastated - FIL has always been his hero, and now he has no idea who he is. DH feels very strongly that he wont lie/cover up etc but can't bear to break MILs heart. It's all such a mess, and he's stressing about the devastation that will ensue, or that MIL will hate him for exposing the whole sorry mess. Likely that MIL has no clue, FIL's job gives him a lot of opportunity to get away with it.