[Biddy44]

I'm a left-hander who plays guitar righty. As others have said, the pool of instruments is larger, and you can borrow someone else's at an impromptu gig. The guitar I picked up first was a righty, and off I went. Personally I think the work of either hand is challenging enough that neither way of playing is the 'right' or natural way.

My advice: make sure she's comfortable in her decision, and let her go.