The last thing I would want to do is to try to tell the guitar player how he should shape his tone..... He's setting it the way he is for a number of reasons that are import for him to feel comfortable with how he hears it on stage and how he senses it blending with the band from his own perspective. Of course, there could be a number of reasons why the placement of his speaker cab relative to his ears are making it sound "bad" off stage, but unless he has the freedom to place his cabinet in any position he wants to, to achieve his ultimate tone (from his perspective), then he has little choice - make it sound good for his ears, or make it sound good for the room and probably suffer a lack of connection and feeling for himself.

If you can suggest a way for him to reposition his amp and/or himself to maximize the good tone you seek, while also keeping it for him, then fine... maybe the two of you can find that solution. Otherwise, I'd say that your solution is going to lie in the previous suggestions given - mic placement, mic vs. direct and/or eq settings at the board. It's doubtful that he'd be offended or put off if he should happen to hear a bit more beef bouncing back to his ears from the room, unless it muddies up or jumbles up the onstage blend with the bass and/or keyboards.