You are leaving out one important thing besides they are still married. Who owns the house/dwelling she is living under? Does it belong to her or are they both mutual owners with the understanding that after the divorce is final she will keep the house/dwelling?
If it is still under ownership of both individuals then he has as much right to be there as her until all the assests are settled? She may also be doing this to keep the divorce from becoming a cat fight between each other and prolonging the divorce proceedings resulting in spending more money than necessary.
In any case, you are still the third party as long as they are still married. So, quit being a stick in the mud and relax.
Hoop( part time dirty old man as recent being accused of)