the situation is like this, said person has an irrational hatred hatred of 4e (IMO, he hasn't given it a fair chance), the rest of us like it, and would play if there were a game going on. Thus he's admitted would not play, but would show up just to socialise. Personally, I feel that leads to disruption and bad vibes at the table, thus my asking here.
In your place, I'd let him go (though I'd ask why he thinks he'll have fun hanging out while others game), but call him on any antics early and often. Do so politely and kindly.
I"ve gamed with roomates, SOs, and other friends around where it worked out fine, but none of them demanded equal attention as the game for that period of time. I worry he may get the exact opposite of what he wants (inclusion).
Also, depending on what kind of irrational hatred he has, perhaps he can be sucked in, or given monsters to play during combat, etc.
Yeah, you have to get it "on the record" between you and him ... is he coming to play and is he going to bring his best game even if he doesn't like the system. If he is just coming to socialize or if he is going to play reluctantly, then suggest he don't come. That's tricky, I realize. After all, he is part of the gaming group, which is also a social group. So, you don't want to totally ostracize him.
The other thing you might try is recruiting him to help you run the game. Could he play key NPC's or monsters for you? That way it would be hard for him to sabotage the game as a reluctant player.
Give him the benefit of the doubt. I'm a patient gm, but if he shows up and tries to sabotage the game, he's out of the game group. If he shows up and occasional mentions how his favorite system does it better, it's fine provided it's only occasionally.
That said, I've played in a lot of games where the system was one I didn't like. It never really mattered to me unless I was running it. He deserves the opportunity to play until he makes an asterix of himself.