In the real world, whether or not it is legally permissible or socially acceptable for a parent to marry their son's ex-wife while the son is still married to her depends on the laws and cultural norms of the specific country and region involved. In many places, such a situation would likely be considered socially inappropriate and might create tensions within the family.
It's important to recognize that family dynamics can be complex and sensitive, and decisions that could potentially affect relationships within the family should be made with careful consideration and open communication. If you find yourself in a situation like this, it is advisable to seek legal and professional advice as well as have open and honest discussions with all parties involved to navigate the situation with respect and understanding.