Do it like this:

1: Look at the exclusives.

2: Follow your gut feeling.

I think you have your answer then.

He'll miss out on Halo, Gears of War, Fable, Forza, Crackdown, and many others -- which is regrettable -- but he'll still be able to play Mass Effect, The Elder Scrolls, Assassin's Creed, Call of Duty, Grand Theft Auto, BioShock, Battlefield, Left 4 Dead, etc. Plus all the PS3 exclusives and all the PS1 classics on PSN.

This won't end well

but go with PS3

Not even a contest.

PS3 is great value for money. If your friend is new to gaming then GOW, Infamous, Ezio Collection.
PS2 Classics in HD, exclusives games, free online gameplay. PS3 will be supported for many more years to come and great games already next year.

Buy both. And a Wii U. And a Wii. A Dreamcast too. Gotta buy them all!!

Personally I prefer PS3.

I love Ratchet and Clank, Uncharted, Resistance, Infamous, Littlebigplanet, Journey, Flower, GT5, God of War, the exclusive HD collections, and MGS4 so obviously I say PS3

If you really like Halo, Gears, Forza (basically the same as GT so I wouldn't let this be a deciding factor), and L4D (although those are on PC).

Some people say the PS3 exclusives aren't good, but I really like them and most of my collection is exclusives. Also you should take into account that Sony said they will support PS3 till at least 2015 and when you take online cost and the fact that if you buy a 4gb 360 you'll most likely have to get proprietary, overpriced, memory before too soon I actually think of it as cheaper if you do any sort of serious gaming. Also if their past systems are any indication, Sony will support PS3 even longer than that and they have a bevy of exclusives coming up next year with Puppeteer, God of War, Last of Us, Sly, and Beyond.

Another thing to take into account is PS+ which for $50 a year can keep you very entertained in dry spells (I haven't bought a new game since Sleeping Dogs and have been keeping busy on PS+ games).

I know this seemed biased but I don't buy games because of the system, I buy systems because of the games. If you have a chance try out the games I mentioned and see if you like the 360 ones more.

What do you and the majority of his friends have?

I'd start there then add another later if there happens to be exclusives on another console he sincerely wants to play.