His problem using the visa when he is divorced is that he shouldn't even have that visa at all. His wife was the principal applicant so I don't know why she didn't just tell them they were divorced. If she didn't need him for points, then there was no reason for her not to tell them. It would not at all have affected her application. He on the other hand shouldn't have this visa. If she would have told them they were divorced, she would have gotten the visa for herself and he wouldn't. That is the problem with using it for him. If it ever comes out that he used that visa, he could lose his PR. Even after citizenship you are not safe if you gained your PR through misrepresentation.
Either of them could get caught if they later re-marry and need to sponsor a new spouse. Immigration may look at the date of landing and date of divorce and realize the problem.