My initial thought is that the "loud and proud" guitars are probably full of midrange tones. This is a very common issue with the search for a "fat" guitar tone.
You said: "Her voice seems to be thin with no real substance to it." - probably because the strong part of her voice is in a range that one or both of the guitarists has in their tone.
So, here's a process for you using channel-strip EQ's:
1. If you haven't already done so, sweep the hi-pass up until her voice gets really thin, then back it off until it sounds OK.
2. Work with the vocalist alone, boost a midrange tone and sweep it around while she sings, listening for the part of her voice that is really awesome. Also listen for any parts that suck. Boost the awesome and pull back on the suck. Narrow both down to the smallest Q that works.
3. After doing that, have her sing with one of the guitarists, then the other, to see if the problem is there with each one alone (or at all - the first two steps might help the situation).
4. If it's just one guitarist, work with them on their tone - the goal would be to have them dip out the part of their tone that matches up with the part of her voice that is "awesome"
5. If it's both guitarists, do the same with both of them. It doesn't hurt to do the same process with them that we did in step 2 with her - find their "awesome" and "suck" and boost & cut. Also, they should both be hi-passed more than they think. In fact, all instruments that are not kick or bass should be hi-passed until they suck, then backed off just enough to not suck.
Don't be surprised if one or both of the guitarists takes offense at this approach, it is really common for guitarists to think that "fat" tone requires a lot of bottom-end. If they refuse to work on tone with you, then they'll get what they get for a mix and nothing better.