I agree to present all the arguments in favour of RH playing, and let her try it.
The problem with playing the other way is the strumming/picking hand will need a
lot of training. Rhythm, timekeeping, precision of attack, is fundamental. There's a reason why all string instruments are played the way they are (RH picking, strumming, bowing, LH fretting or stopping) - and that has to be rhythm.
However, if she really is comfortable playing her way, and it feels very unnatural for her to play the "right" way - and especially if her sense of time seems OK - let her play LH. After all, it worked out all right for B B King, Mark Knopfler, Gary Moore, Noel Gallagher, etc (left-handers who play RH).
