How do I know if microneedling will cause volume loss in my face?

Microneedling does not cause volume loss if anything it  helps collagen production although not significantly. You may want to consider volume replacement with fillers or fat.

its very rare that this would occur, and if pictures were taken it would be evident, but years I have been studying devices and doing these treatments and never seen this., sculptra would be prevenative going forward and I would suggest laser + microneedling and not microneedling RF If she truly feels there is an issue.  Make sure you do proper skincare after treatment as this plays a role in the amount of collagen produdction (see link) long-term and cosmelan/enlighten peels are best between treatemnts to get even better results.