Does the existing hub use a freewheel or a cassette? If it's a freewheel you could remove it, rotafix a cog on, and use a BB lockring and some lock-tite. This shoudl work okay if she doesn't plan to be doing mad skids.
The shop wanted $800 to cut the old hub out and install a new on in the original rim?! I'd never go to that shop again!