My husband's ex tried to take hime to court for going against their divorce decree when he stopped child support for one child who joined the military at 17, which according to the decree he was within his rights to do so. So, they agreed on a set amount of child support for the remaining child. We recently received the paperwork from her lawyer and there was a part added that goes against the divorce decree, namely the part about child support would stop under certain circumstances, ie the child turns 18, marries, joins the military, or dies. She is trying to add that when the child turns 18 or graduates from high school,whichever comes LAST because the remaining child will be almost 20 when he graduates because she held him back twice without discussing it with my husband. Can she do that? What can we do?