What causes raynaud's phenomenon? My friends hands turn comp blue with the slightest bit of cold. Doc said it was just rs and was ok

OK? 7% of woman have raynaud's phenomenon. Most occurs in isolation, but workup is required to assure not part of a larger problem. This does not just mean laboratory studies, but requires assessment by a knowledgable/experienced physician.
Unknown. This is an annoying condition, caused by the body narrowing the arteries to the extremities when the core of the body temperature gets a little low. It can be caused by excess sympathetic nervous system activation too. The best defense is to keep warm, wear a hat outdoors, and stay physically active. There are medicines which can help a little too.