I would just let it ride. Compress the crap out of it with a transparent compressor, lower it in the mix (it was too loud, which made it really sound worse than it needed to), throw some ambience on it and give it to them.
There really isn't anything that could fix that performance. Autotune will only fix the pitch. The timing, dynamics and delivery will still suck.
It shouldn't fall on you to tell her she sucks, if she figures it out on her own and asks for help, help her all you can.
I would crank up the guitars and turn it into some kind of punk track and get her to scream the vocals. good luck!
We need more details as to how much this matters....