I think the best thing to do with recording music is always to start with what you've got.

I don't know Adobe Audition (I use Logic Express) but it looks like a decent, standard piece of music software. It'll certainly be capable of getting professional results in terms of recording audio, mixing it, EQing it etc. Before you buy anything else, could you let me know:

1. What's the name of the microphone she's using?

2. What's her soundcard? Does it have a pre-amp? And what hole is she plugging the mic into?

Depending on the answers to those questions, I suspect she can probably get good results with what she already has.

In terms of the specifics of recording voice and guitar, you're always best off (from a recording and mixing point of view, that is) recording the guitar and the vocals separately.
It's a pain in the arse of course - it's much more natural and fun to do in one go.

Best thing if you want to do it all in one go is to have more than one mic. I'd probably use two on the guitar and one on the vocals.

But even if you were just using the one mic, you can get a good mix of the guitar and voice by careful mic positioning. A recording engineer would read "she can not find a microphone that gets a good level of guitar and voice" and answer "she hasn't spent long enough experimenting with mic position".
Unfortunately, there's no substitute for the boring process of just moving the mic around, recording a verse, stopping, listening back, moving the mic again, repeat ad infinitum until you've got a good balance.

Start with the mic about a foot in front her mouth. Remember, you want significantly more of the voice than the guitar. (One thing I've only really recently appreciated is just how quiet instrumental backing really is compared to the voice on almost all recordings. 9 times out of 10, when you think the voice is too loud in your mix, it's probably not actually loud enough).

One good, if unusual mic position might be over the left or right shoulder (as if it were a parrot and you were a pirate). It'll pick up the voice more than the guitar (wicg