What he should do is go back to the family court to try to have his ex- held in contempt of court for not refinancing the home, or other appropriate relief. He could also speak with his ex- about his taking over ownership of the house and refinancing the home himself.
If he brings in a tenant without his wife's permission, he runs the risk that she will decide that she wants to move back into the home -- and in all likelihood he would end up getting sued by his tenant.

