What is her reasoning?
In working with my students if I advise them to avoid something I try to give them a reason…. some research, some anatomy, some biological plausibility.
If a student mentions something asking (not in a defensive way, but really to know) both makes them feel that they are being listened to, and presents you as a professional secure in your knowledge, and open to new insights.
I am wondering if her reasoning is not that the grapevine is bad inherently (except for those with balance or mobility issues) but that it is a more complex movement for a warm up. I only teach cardio occasionally these days, but I do use grapevine, though I would probably not use it in the warm up, until I have built up to it with some simpler options. That said, I would not suggest an experienced dance teacher might have a different answer.
It is a no no for anyone with a hip replacement. Otherwise as long as you ease into it (should not be a high BPM song) and the participants have the coordination and balance to follow it should be fine. However, if she is really uncomfortable with it you might want to ask her the rationale. Maybe she has a good reason and we could all learn something.
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.