I guess depending on how the 2 players match up, this might not be such a bad thing. If he's overmatched he'd need something to make it fun, it'd just be alot better to come to your friend before the game and say "you play a few levels higher than me, do you mind if we play with rageing barbs on and i build the great wall?" If i were playing a friend who put it that way i would have no problem doing it. But if they're comparable skill players, then forget it; i'd just attacko him until he quits every time. Also, start keeping score, counting his quits as losses, it'll give you more trash to talk in real life too.
Yeah but I can think of a half dozen ways to handicap that make the game more interesting than raging barbs. Sure you can spawn bust and you can place skirmishers on hills and deal with it, but at the end of the day you face the problem that a bad dice roll sends barb threat spiralling out of control; and let's not forget that the ever popular barb galley fleet of doom.
Further, assuming you have AIs in the game, is he going to quit if the AI takes the GW? Is he going to get better if all the AIs are gimped by barb problems (and raging barbs is sufficient to kill off the AI on immort)?
I'd much rather just agree to gift him some techs, or give him an early worker or even a second settler via WB than play this type of cheese.