How much does it hurt her to use the finger for fretting? If she's not using the very tip, it shouldn't be putting too much pressure on the ouch bit.
Coincidentally, I tore the nail on the middle finger of my right hand a week ago, which made plucking with that finger uncomfortable for a few days. OK now though. Average rate of growth is around a tenth of a mm per day (however, mine seems to have grown 2mm in a week).