So the XLR main outs are being used to feed the PA?- Yes, she has a portable speaker she connects this to.
^ One portable speaker feed from one of the main XLR outs? In that case I presume she's getting whatever mono mix of both vocal and guitar is appropriate out to the speaker by using just one one of the main out XLRs. Is that correct?
In that case, the path of least resistance for making a mono recording that has exactly the same vox/guitar balance as what she is sending to the portable speaker will be to use the other main XLR out into the recorder. Just make sure the pan pots of all channels are centered. That should work as a fall-back option with no changes to the mixer setup, but will produce a mono recording.
[Edit- this is the same as what jefflester is suggesting]Ideally you'd instead setup the mixer to use the two monitor outs into two channels of your recorder, with the vocal isolated in one channel and the guitar in the other. You can then record both channels separately and mix them later. Will probably take some reviewing of the mixer's user manual to figure out how to use the monitor outs in this way, as it seems there are several options for routing signal to them.
Probably need to spend some time with the user manual anyway to figure out the discrepancy in levels that you are getting through the headphone output. Does it sound correct if you plug in headphones rather than the recorder? ..vocals in one ear and guitar in the other with good levels?
[snip..] so use the adapter above, but only use one of the outs to my recorder? Would it be any different than just using the headphone out directly to my recorder with just one cable? I'm assuming that would just be mono then?
As described the adapter would only be serving as a stereo 1/8" to stereo 1/4" adapter. Would be the same as using a single stereo cable with the appropriate termination on either end. Wouldn't necessarily be mono unless that's what's being routed to the headphone bus.
Here's something that is somewhat confusing which you may not be aware of-
Both the headphone out and the monitor outs use 1/4" TRS jacks, but they are wired differently.
The headphone output is an unbalanced stereo output (three wires = tip/ring/sleeve = Left/Right/Ground). It can alternately be used as a two channel line-out to the recorder, which is what you are trying to do. However in that case you'd need to set up the mixer to send a different mix to the headphone output than the main outs, since it sounds like she's sending a mono signal through the main outs to feed the portable speaker.
The monitor outs are also 1/4" TRS outs, but each is a single channel mono balanced output (tip/ring/sleeve = signal+/signal-/ground). You can insert either a TRS plug for a balanced output, or a TS plug for an unbalanced output, but either way each one is just a single channel. You would need to use both of them to get two channels out to the recorder, with the vocal and guitar signals being routed separately to either.
You don't want to patch a stereo TRS output into a balanced TRS input or vice-versa. Review the manual for the mixer, and check your wiring with the notes above in mind.