I have never had any major problems in the shops with an
ECV. However, I do use a three-wheel from an off-site company which is MUCH more maneuverable than the four-wheel
ECVs which Disney rents.
HOWEVER, you have to be VERY situationally aware of your surroundings and consider the wheel-to-wheel width of the back of the ECV to avoid running over either displays or children. I always make sure that in a store I will keep the control dial to "turtle" as well as only very slight pressure on the control lever.
Once in a while, but rare, I will get in a location where I may have to back up a few feet to get a better path around an obstacle.
As long as she is more careful in the shops than in the middle of the street there should not be any problems.