The best solution would probably be as you mentioned, IEMs.
I'm a bit surprised actually, as earplugs make it MUCH easier for me to sing in key. Its like the old holding one ear so the sound vibrates in our skull. If that's NOT happening with her, and IEMs are too complicated (which they can be), this is the best I can come up with...
First, have her try different earplugs. Either the big fat thick drugstore kind (they make my skull vibrate most), or musician types that can be found on Amazon for not a whole lot of money. They allow for a lot more frequencies to enter. I found a decent set for around $25, after a lot of research. I'd also check how she's putting them in her ears, as I've noticed some people have strange and interesting ways of doing that.
My second, and probably better, suggestion would be to make sure she has a monitor next to her that she can hear the guitar clearly. My best guess is that with the earplugs in all she's hearing is drums, and that's what's causing the problem.
Hope some of that helped. Its all I've got

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