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.