This is what we all want, but it's hard to achieve and quite contradictory.
Considering he has no special interest, I always recommend Creator sets. They have the best play/build/number of bricks - rate.
Here my suggestions:
Creator:
7291 - Street Rebel
or 7292 - Propeller Adventures (both 20$)
depending on if he's more interested in bikes or planes.
Star Wars
you only get small sets but with cool minifigs. I'm not sure if that is, what you're looking for.
Anyway - my recommendation: 9488 - Battle Pack (13$)
Talking about Amazon it's always a clever idea to look out for discounted sets. You won't receive more bricks with buying regular sets instead. So grab whatever you like for 20$:
The toy story set 7590 is great and you may call it an icon from the first TS movie.
6866 is a good set as well, but comes with >40 bricks less than the Creator set 7292. Here you once again get cool minifigs (licensed set) and some really nice dark red bricks. It all depends on what you or your friend prefers.
I checked Amazon.co.uk, here's what I found else:
City
3221 - Big truck (discounted 12,99 GBP from 29,99 original price!) - Fantastic set. If your friend is somehow into City, this is a "must buy"!
Technic
8067 - mini mobile crane (14,99 GBP instead of 20,99) - a good start with Technic and a nice set with cool play features
Well - it's hard to spend money on LEGO wisely...