Paint the whole belt buckle with clear fingernail polish. It will sort of "seal" it and keep the nickel away form her skin.
You will have to refresh the polish every once in a while.
I am allergic to nickel and I do this with the posts of pierced earrings that have nickel in them. It works.
ditto. I am allergic to the nickel on the underside of my watch. I put clear nailpolish on it and my wrist is fine. I have to reapply every so often but it beats the alternative (a rash).