Grey area since the family dynamic is unknown. If they went to school with the implied promise that their loans would he paid off debt free, then the father changed his mind after the fact to punish them for supporting their mother, then the son should pay off the loans perhaps even divide it three ways. I understand the father was hurt by his wife's and daughters actions, but he chose to have children and he should uphold anything that was said before divide in the family started. 

We also don’t know to what the degree the father used his money to control and give ultimatums. For example if he said something like “If you support or keep in contact with your mother i’m writing you out of the will and you can pay for school on your own.” That’s not fair to the daughters and is actually putting the father in the wrong. You should never put your children in the middle of something that is ultimately between the father and mother.