A lot of what is going to happen depends on the original order. If there is no school tuition mentioned in the original divorce, chances are they will never require him to pay it now. In order to get these things changed, she would have to show a substantial change in circumstances that could not have been anticipated at the time of the divorce. For example, to go in and try to raise the child support amount because the kids are older. That is not something that was not anticipated. But, if someone's wages substantially change, then she can go to court for it.
In Illinois, courts can order educational support regardless of the age of the offspring. Support applications can be filed before or after the person reaches the age of majority, and orders can apply to expenses related college education or other professional training after high school. By law, expenses include room and board, academic fees, tuition, transportation, books, application costs, medical expenses (including insurance), and living expenses during the school year and recesses. Judges must consider the financial resources of the parents and child, as well as the standard of living the child would have enjoyed had the parents not divorced (750 Ill. Comp. State. 5/513).
The $3100 can only be collected if it was something that was ordered previously but not paid. In other words, if he fell behind in his payments. If it is just something that she wants that was never ordered before, he will not have to pay.
I am only assuming some of this, since I don't have the original court order here. But, as a general rule, you can collect what was ordered by the divorce but not paid, but you can't collect a back amount that was never ordered anyway.