"We haven't discussed this yet, I will be seeing her next week and brining it up. "
Start with that conversation.
If appropriate to the music and if she is good enough to come across well "fully acoustically" as one would hear her singing and playing without any amplification, then what grawk suggests is probably the way to go. Few mistakes to make, things to screw up, or processing to be done, and the possibility of capturing something excellent - simple, pure and delicious without the recording process getting in the way.
If she wants to convey the energy and feel of a live performance. Do that at one of her gigs with and audience of supportive friends. Set up your simple stereo pair of microphones beneath her vocal mic on the same stand. Great live recordings of solo singer/songwriter type performers have been made that way.
If she doesn't want a live feel, do it the same but without an audience, preferably somewhere with good reverberant acoustics. Be creative in finding and using those places like that.
If you have to, do it the same way in a walk in closet with lots of cloths hanging in there, or something similar to deaden the acoustics of a small room. That will sound very flat and dead and will need added reverb. A large room with good acoustics will sound much better and more natural, which is what makes that the better option.
Recording her the same way in in a typical untreated garage or room in your or her home can certainly work, but won't sound nearly as good as a decent sized space with good acoustics, and is likely to sound worse than recording in a very dead sounding closet like space with added reverb. The sound of a small room tends to clutter up this style of recording with too many early reflections and no good reverb. The resulting recording is going to need added reverb anyway and the not so good small room sound will remain.
The effort to find a decent sounding space will be worth it, and this simple recording method makes taking advantage of that kind of resource easy.
Recording in the way Chanher suggests is the way most folks are likely to think about going about it, but requires more setup, experimentation to get the mic setup right, mixing work, all that jazz. Quite different skill set than concert taper style recording. It can make for a produced-sounding recording, but will require more skill to pull off well. This sound might be her expectation, however I'd steer you toward what grawk suggests first if that can work for her. This method will take a lot more trial, error, learning and effort on your part, but can better support things like effects and compression if the music requires that kind of thing. It can also add some polish to a performer who can't pull off the simpler straight live to a stereo pair of mics thing as easily. Once you do figure out a reliable and good sounding systematic approach for recording vox and guitar in this way, you can then more easily adapt this method for other musicians who perform with a similar style.
If you want to offer this kind of service regularly, it will be beneficial to be able to use either of these approaches, choosing whichever best fits the performer and their music.