I think I end up rescuing more horses who someone had purchased thin and rescued themselves, thought the horse was quiet, packed pounds on and then they couldn't handle the horse.
Its a fact of life. If you had worms, hooves in shabby shape and you were underweight you'd be less than playful as well :)
The second fact of life is if the horse is in this shape you shouldn't be riding it so "paybacks are a ..." well you know!
All-in-all however I think the miss-behavior has much more to do with the new owners than the food and health up grade in most cases. Knowing this from personal experience, I've worked with many rescues and while yes, they perk way up once fat, happy, healthy again... They DO NOT turn criminally insane on there own
They do not go from bombproof or quiet to nuts if properly handled and worked with, they simply kick up there heels more often in the pasture and become more alert and themselves again. If treated wrong, the energy levels go to there heads and they test the new owner, who may not have bargained for the FAT version of the THIN horse they bought.
Simple, common problem.
Can you have a trainer work with her?