#1) if the 2nd owner did not finish paying for them, than the first owner should have reported them as stolen property. If there is no police report she is going to have a hard time "proving" anything at this point.
#2) If she has the real first and last name or address of the 2nd owner there is no reason the police could not have re-claimed the horse or tracked down the owner.
#3) If she was so concerned, she would have taken the 2nd owner to court.
Now if she has filed a police report and listed the horse as stolen, technically you are in possession of stolen property and she can reclaim the horse. If she tries that, be sure she has the police with her- otherwise she could be "faking".
If the horse is stolen, it is also possible that you can pay the original owner for the horse and the papers, but you would probably need to take the 2nd owner to court as well to get your money back.
It may not hurt to send her T's information- as that is the person she will probably have to take to court to get her money back. Again a very messy situation...
How much is the horse worth? If this is a high value animal I would be more concerned vs if this was someone's backyard pet. It sounds like she is more concerned with the money she is owed than getting the horse back.
If she does not have a signed contract with owner #2, or a police report, there is not much she can do legally. Check netposse and see if the horse is listed as stolen.
I doubt she has the energy or means to go after you or the original owner. Unless these are high value animals it would probably not be worth it to go to court.