Follow Mac's advice. It makes perfect sense.
If the signal distorts on its way into the computer, there is nothing a realtime effect can do to change it. The actual conversion from analog to digital signal will contain the distortion already.

Having a low vocal signal will be MUCH easier to fix.

Don't let her hold the mic, have it on a stand - this will help with volume fluctuations from mic movement. Some singers tend to bring the mic in closer when the sing louder, instead of moving it out like they should, and this would only compound the problem. Bringing the mic in is a natural reaction from the body tightening (contracting) to produce the louder volume, and takes disciplined practice to overcome.

Make your sound your own!
.. I do not work here, but the benefits are still awesome