My band recently did a show and got the recording of it. It's the best quality live recording we've gotten so far, but in some of the tracks the guitar is a hair too loud. Is there any way I could lower just the guitar in the mix? I don't need to turn it down too much, just a little bit.
Not that I know of, I know I've been out of the recording business for a while, but as far as I know, you have to have each instrument on a separate track to control the volume of the instruments. Recording live completely relies on your stage volumes.
Not enough detail to say. If the sound guy just took a stereo desk feed (which is how you make it sound), that's it. However, for all I know the engineer tracked each channel and what he sent you was a mix. Sounds unlikely though.

There's certain tricks you can pull to try and improve things (mid/side processing, multiband compression, dynamic EQ etc etc). Slightly taming loud guitars is do-able, especially if they've been panned out of the centre.

You know what would really help? A clip.