He is if the divorce decree states that - as long as the child is in school and up to the age of 23.
You are wrong and your ignorance of the matter shows. Before you give someone advice you should check the facts. The age of 23 obligation would depend on what state the divorce occured. No, in my state you are only obigated until 18. I was helping out of the kindness of my heart and the fact that she's my daughter.