child visitation execution petition filed by the mother

Our son and daughter in law seperated in 2017 by a mutual divorce. Under this settlement my son has the permanent custody of the child and my daughter in law was given visitation. She did not exercise her visitation properly, even though my son requested her to come and see her 5 year old son. She was very busy with parties, djing, and she also does drugs.We have pictures from her own facebook account, spending time on the visitation days away with her friends. Now, my son is re-married and my ex-daughter in law after two years has filed execution petition for visitation. My grandson was looked after by all of us for so long and now he is 7 years, my son does not want to give her visitation rights. What would be the best thing to do?