I went thru hoops with my first horse. The lady's daughter's horse. She lost interest, and aside from trying to find people to ride him while she paid board on him, she was looking for someone word of mouth.
The lady said if I want to buy him, I would have to split board on him for a few months. She finally relented after 2 months, and let me buy him. He was a joy till he died of old age.
You might want to try calling and asking her if you can spend time with the horse at her place anyway even if she's keeping him. It may get her to come to her senses after a few weeks.