I'd be worried that come January she'll see what great shape he is in and want him back, or want to sell him to you for a lot more than he would have been worth when you started leasing him.
I would make every effort to have this settled with you as his new owner before January rolls around.
This would be my concern.... and I have had it happen. Only it wasnt a lease. The guy gave the horses away. Once we started trying to rehome them when they were healthy. He saw how nice they looked and wanted us to pay him for them. But since we had pictures of before we got them, when we got them, and every month of their recovery. PLUS... all the medical records... his attorney told him itd be better to walk away because if they saw the shape they were in under his care... he would get in trouble.
So if you have all those records it will help you out alot. And if you have pictures of the then and now.
Many people have sighed for the 'good old days' and regretted the 'passing of the horse,' but today, when only those who like horses own them, it is a far better time for horses. ~C.W. Anderson