I think it would also be highly advisable for your mother-in-law to contact the 2 main credit referencing companies: experian and equifax, asking for a statement to be put on her file, give the facts, Her ex's full name, date of birth etc, that your mother-in law and her ex split up on X date, have been divorced for x years and he has not been living at your mother in laws address since X date. Also that their financial affairs are completely separate and that no credit should be given to said ex in a joint name, or with your mother in laws address.
It may mean that if your mother-in-law applies for any type of credit, approval may tale longer as the note will trigger a manual review, but will mean any of his applications should get through if they implicate her or her address.
Worth doing as peace of mind.