the way though the manga presents Roxanne's eye restoration seems to make it sudden and completely not Roxanne's own direct doing, as she seemed to be shown completelely surprised and caught off guard that her damaged/missing eye is just suddenly back and/or undamaged.
but, maybe the manga was jsut being quick about it, and not giving us the full detail of her returning eye too.
I have to lean though on the side that it's something magical,
something similar to Hidan's Jutsu in Naruto... I'd love to see what would happen if a "befriended Claymore by Roxanne" kills herself, would Roxanne die too?