The high withers aren't this horse's problem. The neck ties in too low and lacks musculature. I'm not fond of this horse. She isn't balanced--too uphill.
I did a quickie search here and found this thread: Before and after( muscles!!)
Look at the pictures on post#3 and you will see a more balanced withers-neck picture, which the mare you are interested doesn't have. (The weight difference between the one in your post and the other horse isn't important.
I always suggest that someone buy the best build/best temperament possible. Temperament trumps build but unless your grandmother can safely ride this mare, I'd keep shopping. It's still a BIG buyer's market. Be patient. =D