My friends son in law is having an affair with the her in which she is the grandmother of his kid , how would that affect his rights of shared custody

I'm not trying to be rude or be judgmental in this situation however it really sounds like your friend AND her son in law need some counseling. This is not a healthy relationship as who cares about custody what about her daughter and her feelings? What an awful betrayal .. I hope the whole family can get some help. Good luck ..