I think most of us here have played in his condition. I did last year, with a pretty high fever. If he loads up on asprin or Tylenol, he'll be fine - and happy he did the gig. He'll have a war story. I'd suggest having a talk with him. For his sake, not even yours or the rest of the band's.
And if he still decides not to, then you guys can still have lots of fun, AND learn that you can get along just fine. Nobody is going to miss the second guitar except the people on stage and 2 geek musicians that show up at the show. Unless of course he's the only good looking guy in the band.
Funny, last night I went to see the band for which I sub (extremely rarely- only when he has another gig with a pet project of his) and the bassist said "you know, I was just thinking the other day what would happen if I got really sick last minute, like Im puking or shaking so bad that I physically couldn't play, could you just step in and play all this?" This is 25 years into this gig of his. LMFAO. He's just contemplating this now? He IS super chill. As they used a sub only one time besides me, this implies to me that he's never been so sick that he had to miss a gig in 25 years and they easily gig 100 shows a year. He's either really healthy, really tough or just defied the odds. Now I want to ask him for the details. I'm guessing that he's had to tough it out.
By the way, the answer to his question of whether or not I could play all the stuff last minute was "no way." Oh, there's a new thread topic, I won't derail further here.