Back child support for child over 18

My husband has 2 children with his ex. The children are now 20 and 22 yrs old. Both children are in college. His ex is furious that we do cannot afford to help put the children through college. Recently, she threatened to take him back to court and sue him for the cost of putting them both through college and for "back child support". She doesn't think that he paid enough when they were under age 18. He would love to help both of them but we are financially unable to do so. The dissolution agreement did not mention support through college, only until age 18 or graduation from high school - which he did. While he did get a couple raises, the support was not increased and he NEVER missed a payment.
Would the court system force us to pay for college costs?... and can she sue NOW for the difference of what support may have been, after his raise?

.......we live in Ohio.

[tbyte]

I do not think Ohio mandates support through college, so she probably does not have an argument there.
Her chances of getting back support would be very slim. She'd have to show that the father concealed or misrepresented his income.
This sounds like nothing but bluster to me.

Thank you so much.
...and no, he did not misrepresent his income.