You are mixing family law matters with ordinary civil matters. The family law issues are between your daughter and her ex(s).
Your recourse is to sue both your daughter for the reasonable value of the rent, or for breach of the rental agreement, if the deal was for them to fix the place up in exchange for use of the premises. If you sue only the ex, he can sue your daughter for "contribution," and offset one half of whatever the court awards you.
I understand your annoyance, but the reality is that the court will not treat your daughter's occupancy as a gift, and simutaneously treat her ex's occupancy as a lease.
It will be all gift or no gift at all for both of them.
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Lawyer
Retired (mostly)
Yes. You can sue for any/every thing arising from the use of the property.