Well, it's pretty Disney-esque, but it has history behind it as well. I'd prefer
Bella, but
Belle is fine too.
There are all kinds of names that it could be a nn for,
Annabelle,
Isabelle, to name a few. But let's assume she wants to keep it
Belle. How about:
Belle CharlotteBelle MirandaBelle RosalindBelle AntoinetteBelle JosephineBelle CelesteBelle CatherineBelle Bookandcandle
Belle HannahBelle SophieBelle AureliaBelle Aurore (too princess-y?)
Belle MatildaBelle Frederica ("fred-REE-ca")
Belle AmeliaBelle CarolineBelle ClementineBelle HenriettaBelle CordeliaBelle Georgine