I imagine it would be difficult for even a medical professional to diagnose this from "afar", especially with so little information. You might look up Raynaud's phenomenon; I've known a surprising number of hikers who have that, but I believe it isn't just hands but also feet --- were her feet cold too?

And was she wearing gloves or mittens or any sort of handwear? Was it windy at all?

It certainly couldn't hurt for her to get a pair of decent mittens (not gloves). It also might not be a bad idea to write down complete details of this --- especially if it recurs --- and bring it up at her next physical exam with an actual doctor.