Okay - your OW falls squarely into the psycho predator category. I'm willing to bet she DOESN'T have cancer but just made that up, and that she staged the burglary too.
Not sure what your best approach is. Sometimes CALMLY pointing out that the OW is lying to manipulate him will work. He'll deny it, of course, but it will plant the seed of doubt and eventually he will discover the truth and be mad at her. But the more you attack her, the worse you look, so you need to stay above the fray, above reproach, and be very gentle in how you phrase things.
Hang in there. Whack jobs like this OW will usually do themselves in eventually.
