Abuse of custody schedule?

Not being a person who went through a divorce, I am uncertain of what advice to offer to my daughter. She is going through a divorce, sadly, and they have not had anything come before a judge yet. Currently, they share their daughter, and have a schedule. The problem arises that my ex son in law is always needing to modify the schedule, and my daughter has every time worked things out to accommodate his needs. Now, he has requested a three week block of time to take some personal growth cl. My daughter offered to accommodate all days but four. These four days are days that she had off per the custody schedule. She is using the time for a much needed long weekend break, and if I could keep my granddaughter I would, but I too will be away on a seniors cruise. Her ex to be told her "I am going, so you figure it out." There is no court order to ert, and she feels that she now must give up her time, or he will paint her to be the negligent parent. What advice if any could anyone offer to someone in this situation. I worry because I think this "I do not care, you figure it out" attitude on the part of my ex son in law is going to be an ongoing problem.