This is a weird one, it could even be used as a cheat (in BG2 there are some good magical weapons, it might be interesting having them forever). anyway, i think youŽll have to cheat to remove it, try gatekeeper to change her current weapon, or hex-edit it.
if you need more help tell us
------------------
"Sigh" -Ertai, Captive of the Blinding Angel