It all depends on the divorce decree. If somehow the house did not get addressed in the divorce decree then she would still be 1/2 owner of the house.
Even if it is addressed in the divorce decree, she is 1/2 owner of the house.
Now, if the decree says otherwise, then ex-hubby could sue to force her to surrender her 1/2, but remember - the divorce decree does not override other legal documents. A deed is a deed is a deed no matter what the divorce decree says.
That said, the decree should have wording that forces her to surrender her share under some terms.