My suggestion is just let your son talk straight with your DIL and her parents that he is not interested to live with her (if he is truly not), citing the reasons. Don't mention about divorce. Don't reveal that your son wants or needs it. They too cannot keep a daughter at their home without remarriage, so they will push for divorce too. When both parties intend on divorce, road for Mutual Consent Divorce is paved and it will be easy.
If you show desperation or urge for divorce, they will squeeze you good for alimony.
Good luck,