It's very important to remember, above all, it is a gameIT IS A GAME. The worst possible thing you could every do is take "totally fun and awesome Friday night game night" and turn it into "forced, no fun, role play night"
First explain that in this group meta-gaming is part of the fun. You want to throw cheesie-poofs, laugh out loud, and make silly decisions based on out of game reasons. You find that fun, and you shouldn't give it up. There is no right way to have fun.
Second, for a while play games that are close to what you normally do, but take you out of your context. Play some Munchkin for example. It's kinda of like role play (not really) but it encourages "meta-gaming". The general idea is to create a totally different experience and "shock the system". Heck, if you really need to, break out Monopoly. It's about the social gathering, not the Role playing. Obviously, you want to get back to role play, but when this new person is more accustomed to the group they may be more open to adapting their play style. Don't fear other games, to break the ice (or to insert a pause).
Finally, if all else fails, ditch the new guy. Sucky social interactions suck, but you have a good group, that group is having fun, protect that connection. It's what it's all about. Nobody (well not many any way) buys 20 sided die, and massive tomes of rules, and pre-printed character sheets because they think they're going to be able to sell their epic D&D loots to pay the rent. They do it because it's fun, it's social, and it's entertaining. Don't lose that. It's (the social connection) the most important thing in the world.