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.
ETA:
Just read you have ended the lease and found two others to ride. Good luck.