I say Cybook since it has a global warranty, and if he/she has access to the internet mobileread, project Guttenberg (I think that's how it's spelt) have tonnes of public domain books s/he can read for free. The US credit card/billing address only applies to the Kindle and Amazon, I'm pretty sure places like BooksonBoard and Fictionwise accept UK credit cards (I know they accept US and Canadaian credit cards) so finding books to buy shouldn't be a problem.
I own a Sony 505 myself but wouldn’t recommend it in this situation because the warranty doesn’t extend to most countries so if the reader were to break it'd be a hassel to ship it to the UK then to the Sony store then back to the UK address then to your friend... Hope that helps
