Previous lesson horse crow-hopping. What direction to go?

I’m an adult amateur free-leasing a 15 y/o QH mare who was previously (5 years ago) lesson horse, with a previous owner.

Her current owner only has bits of information.
Through my network, I’ve found 2 people who rode or previously. Both agreed she’s a sweetheart, but that she developed “problems” after some unknown incident that she injured her hip. Since that incident, she developed problems with cow-kicking at the canter (but only under saddle) and crow-hopping. She will WTCG without a rider with no problems.

Now, I’ve ridden this horse maybe 10 times. The first two rides were bareback in a halter, like I wanted to make sure she was even rideable. She was an angel.

Then I brought in my trainer and I’ve had a couple lessons on her with and without a bit. She’s was fine for both. One we asked her to do some real work - bending, leg yielding at the walk and trot. She was still great and really put in the effort. We’ve also been doing some natural horsemanship.

Then out of nowhere I go to ride her again and now, after just 5 minutes under saddle she’s pinning her ears and crowhopping, complete refusal to move her feet. I get off and do groundwork. Same thing again.

My trainer says that she is just a mare being a mare. But the time/effort I want to invest in her should be considered because she is not my horse and this could be a long process.

The owner is relatively indifferent, although he d prefer that I continue working with her. He does not ride or know much about training.

Bodyworker came out and said the horse is just a little sore from being a lawn ornament for 5+ years. No major issues noted from the vet.

What do I do? Hire a trainer to work with her? How do you address this problem (knowing that this is NOT my horse, I’m not paying a dime to ride her (but I desperately want to be riding her!!), I don’t have the know-how to train or the funds to invest.