Not a female, and nowhere close to being as good as you described her, but if you can convey it to her, please do - practically no one cares (maybe not the best way to put it). What I mean is, I have never looked so closely at a women (or their legs). I have other male friends, who are also attracted towards females, and they have never noticed (or at least have never discussed this detailed noticing with me). If she is uncomforable, there could be 2 possible reasons, either she thinks what others think of it, or she is worried about it from a medical prespective. I will come to latter later, but for the former, as i said, I do not think anybody really pays this close attention (at least I do not, and i consider myself as average). Maybe you would have noticed, potentially because you are their partner or close friend, but very likely even you might not have really noticed till she mentioned it to you. So please tell her, it doees not matter.

Medically, it is perfectly normal to have your veins being observable. My guess is that probably the person has really fair skin (almost white), and may not have many melanocytes (maybe genetics, or maybe some disease), and if she does not have a mild or severe reactions (for example, itchiness, or redness) to going out in sun, then her body is producing close to adequate amount of melanin. Veins do appear if the skin is thin around those portions, and skin itself is light (absorbance). If they do not have any reactions in going out, she should feel perfectly ok (tanning does not count as a reaction, that is expected, also, she may not tan much because of lack of melanin, but that is a separate thing). If she has some reactions, then she should consult some dermatoligist, and should probably start wearing some creams (some appropriate spf, sorry, I do not know much about this stuff).

TL;DR - Tell her to not care. In case of any allergies, consult a dermatologist.