even though just because something has been used for the longest time is not necessarily a reason not to question it, this is certainly a strange comment. Lateral moves are standard in all group exercise classes, and the grapevine is just one of them particularly in a ‘regular class’ which is not for special populations.
I would ask her reason, though. It seems obvious to me that she has difficulty with it else she would not question it. If she has problems with other lateral moves as well, you might suggest (as tactfully as you can) that this may warrant special attention as this may be an early warning sign of an underlying problem.