You need to consult a lawyer not a forum of horse enthusiasts.
Laws are different state to state, country to country regarding livestock.
To me, if she has undeniable proof said horse is her horse, a court order to take possession of said animal then she should be fine to take the animal.
Where it gets sticky is the horse is on private property...and it doesn't sound like it is the property of the person holding the animal "hostage" either.
Where my mind goes to though is...
If the person providing all this information about the animals location knows, has seen and has access to ...why is this person not helping by taking said horse for a walk and graze time and hand the animal over to the court-appointed owner?
Sorry, the entire thing smells rotten and some piece of the puzzle is yet missing.
Make sure the law is on your side = pay a attorney to answer your questions about your legal rights, the rights of the land owner who will be trespassed on and the rights of the person who is in possession of the animal currently.
Agricultural and livestock laws are far different than many other laws...you must
know your rights and what is legal or not or you face jail, fines and a criminal record.