From a legal standpoint, yes, it really matters what it says.
If what OP says is accurate (and we still don’t know whether she has seen or quoted the actual decree), the intent seems to to split the actual cost fairly between the parents, up to a maximum of the actual cost at MSSU. The school was presumably named as the standard so that there couldn’t be any huge surprise costs. I repeat: the intent seems to have been to split the actual costs, up to that limit. That is not unusual; you don’t want to agree to what you think is half the cost at State U and then get handed a bill for half of the cost at Ivy, so you put the limit in the clause on what the parents must pay. So even if the child does get accepted to an expensive school, neither parent can be forced to pay more than the decree says.
My arguments are an attempt to predict the legal requirements, based on the information we’ve been given. I don’t know his finances; I don’t know any more than anyone else here does. But IF what OP said is correct, neither parent is legally obligated to pay more than what MSSU costs (zero).
Yes, Dad and Mom were both lucky that she earned a full ride and they didn’t have to pay anything out of pocket. Dad and Mom get to split the zero cost. Yay for them!
And according to OP, he has paid what he is required to pay and is still paying full support for his other child. Sort of the literal opposite of a “deadbeat.”
Would it be nice if he helped her out? Sure it would. But my opinion is not on whether it would be nice; my opinion is on whether he is legally obligated. And from what she has told us, I think he would not be legally obligated.
Does she really need help? NO. She wants to transfer, but she has a full ride that she can continue to ride out for two more years. She does not NEED Mom or Dad to provide money.
My view (this is not legal advice) is that trying to get money out of Dad, on the facts given by OP, would be a huge waste of time, effort, and money (attorney’s fees), accomplishing nothing and aggravating what is already a difficult situation for all of them.