This is a tough one. Is she on the deeds to the property, that is the key in all this and not necessarily the mortgage. If your son can prove he can manage the mortgage on his own then I dont see a problem.
However, if she is on the deeds to the house, then she is entitled to a share in its equity. I dont mean to sound negative but knowing what a friend went through, it was all very long and drawn out and cost a large amount in solicitors fees. Are you sure she can get legal aid?
What are the legal department saying? I know that if it went to court, the chances are he would have to pay her something but I doubt she would have a 50/50 cut in the equity.
Unless she plays ball, there is a chance this could be a long drawn out battle.