I prefer a poly base for wicking. I have wool base layers, but they are so hot, that I never wear them, even when it was -50. My poly boxer/briefs keep me nice and dry all season long, I think they have a very small % of spandex.

On a side note, has she seen a doctor? I assume she has a form of urticaria? It can be induced by several factors, including many diseases and medications. Usually it is cold in general that aggravates the condition, not just water, though it most evident with water exposure due to the rapid chilling effect. I would imagine a light, tight-fitting poly base for her legs would help a lot if the condition is indeed not related to a physical cause or medication.