Having been mostly a GM for 35+ years, when I do get to play, it is always a problem. I constantly have to keep a rein on myself to not go into GM mode. It's tough. However, if you don't step on yourself, you end up with exactly the situation the OP has described.
MOst GMs are used to leading, so when they are a player, it comes naturally for them to go into commander mode. The best way I avoid this is with roleplaying. The most fun I ever had as a player was playing Toronto, the gnome lackey of the Great Gnome Ranger(TM).
But someone will have to speak to him. A warning that you'll quickly lose all other DMs if he doesn't tone it down. One way to possibly placate him, if he is the resident rules lawyer, is for the current DM to ask his advice on rules and/or rulings. Then the DM can agree or disagree, as he sees fit. "Yes, Alpha, you're right, but in my campaign, we're going to go this way...." And frankly, I expect there would be few enough of those situations, as long as his rules are generally fair.
If he criticizes the # of monsters, the tactics, that sort of thing, someone should say - in a friendly tone - "Dude, back off. You're not the DM, and this is how Carl gets experience." or 'Save it for after the game, Carl will probably appreciate some insight, but NOT NOW!"
Another, very good option - the alternate game should be in an alternate system. One he does not know. Star Wars Saga system is the closest thing WOTC has done to PF, and they even have a 10 adventure pack that is free to download. All the way to 20th (the only thing wrong with Paizo adventure paths, they stop just when they are getting good). And the rules are just different enough that when he tries to correct, you can come back with 'This isn't PF, it doesn't work that way here."