I just came back with jamming with a guitarist for the first time. He had some material written. It was very strange. There is no way in hell you could put a click to it. Odd time signatures, multiple tempo changes, just weird. As an example, he had a section that was 5, 7, 5, 7, 8, 6, 6, 6, 6, then back to 5. There was another riff in 7 where he wanted the snare on 1, 4, and 6. None of it made any sense.
I ve been in a similar situation like the author of this original post. Mine was a piano player at a church. The bass player and I would constantly play to him so the song wouldn’t fall apart. The piano player would go off on a tangent and we d gently nudge him back into reality. He wasn’t arrogant. He just was inexperienced. Sounds like your guitar player may be the same. I’d say talk to him and give him metronome. If he doesn’t want to improve it ll be an up hill battle. Good luck!
Just my 2 cents.
Many guitarists perform on their own, so they're not used to an ensemble situation. They also didn't grow up playing in a school band or orchestra. (Same goes for piano bar singers/players), so they've never had to think about balance, timing, group dynamics, etc..