There's actually not that much to watch out for, to be honest. Try and ensure your friend buys from an Apple Store, if possible, as that may make any return/warranty issues easier if there are problems (although I imagine it won't matter that much).
A few points:
1) You will have a US English keyboard rather than a UK English keyboard. Is this likely to be a problem for you?
2) Re: the power adapter, if you have any UK Mac power adapter then you will already have the slide-on plug that you need to use the US adapter in the UK (all Macbook power supplies are 110V-240V).
3) I strongly recommend getting Applecare. Yes, it's an extended warranty, but it is worldwide and you will be able to have your Macbook serviced at any Apple store in the world. You can get Applecare in the UK, when the Macbook returns, if necessary.
4) Your friend might want to take the Macbook out of its packaging for the trip home, then mail the packaging and receipt to you... just in case he gets stopped at customs and wants to pretend he didn't buy the laptop whilst he was away.
5) You'll have no problems with the OS and languages (aside from the keyboard point above) - OSX is natively multilingual.