If you find a good enough deal, I'd say definitely go AM2 for the future proofing. But 939 would definitely be cheaper.
Of course if you can go AM2, you'll be able to use those components toward the next build as well so you would be basically contributing that cost of the next build.
BUT if your friend isn't going to be upgrading at all in the foreseeable future, you'll get more for your money if you go 939