When you say EX pocketed college funds what funds do you mean--UTMA, 529 ,CESA, Trust etc --?
You use the word custodian--which suggests a UTMA or CESA ? The owner of a 529 is free by law to reverse the gift, its a very clear concept--perhaps not what you wanted to hear. Its the owners right!
From the Fairmark site, it would suggest that to convert the UTMA funds so as to meet any support obligation is no illegal . "A custodial account under the Uniform Transfers to Minors Act can be used to pay expenditures for the benefit of the child. It's frequently said, incorrectly, that the expenditures cannot be for something that is included in a parent's support obligation. The Uniform Act plainly says exactly the opposite:
A custodian may deliver or pay to the minor or expend for the minor's benefit so much of the custodial property as the custodian considers advisable for the use and benefit of the minor, without court order and without regard to (i) the duty or ability of the custodian personally or of any other person to support the minor, or (ii) any other income or property of the minor which may be applicable or available for that purpose."
So if Dad can show the funds went to kids use he may be OK.
If he merely pocketed the funds he may be on wrong side of law--but I gather this area of law is not full of enforcement resources at all.
Look up your state's UTMA--it most likley has a clause wherein the child can demand an accounting from the custodian. Child or his guardian would have right to proceed against custodian for improper conversion.
(His kids or your kids ?)