I think not!
First things, first…
Only an offer can be revoked and only prior to it being accepted (save a few exceptions) but not a contract which can be rescinded, cancelled, or voided and only for a variety of [legally recognized] reasons and under a variety of [legally recognized] circumstance. And yes, a "verbal agreement" or contract is just as valid and enforceable as one formed by a writing.
And what exactly is your soon-to-be ex-husband going to sue your daughter for anyways, and on what basis when she has not breached the contract in any shape or form? Well the answer would be: Nothing; Zilch; Zero; Nada; because the last time I checked, Falling Out, Father-Daughter Arguments, and Being Ticked Off were not valid causes of action and his "demands" are simply nothing but an unjustified attempt at strong-arm harassment of your daughter.
You and your soon-to-be ex-husband are, for all intents and purposes, her creditors and as such cannot make such inane demands (not you of course) as long as she keeps performing under the terms of the contract by making the monthly payments at the right time and in the right amount.
He can get on her case if she should ever fall behind with or miss any payments.
fredrikklaw