I don't understand why, if your friend thinks you are too heavy for her horse, she doesn't end the lease herself.
At 200lbs, I would agree you are too heavy for a 15hh narrow horse. Why don't you end the lease and find a horse that is more suited to your weight? 200lbs is not a huge weight for a horse, but you do need to find the right sort.
I expect that your friend desperately wants you to lose weight and is a bit bothered that you may be damaging her horse, but hasn't the bottle to actually call time on your arrangement. Not the right way to go about things IMO.
Just read you have ended the lease and found two others to ride. Good luck.