Not necessarily cheap, but if he has one of the ZVex amps, that's about as small of a guitar tube amp as you can get. (I've noticed a lot of guitar shops with have one or two zvex pedals on hand, which never seem to sell next to the $30 Boss DS-1s.)
Fender Champs, Princetons, and the various Blues Junior models are all great little amps for the money, and if you want that American sound. The bad thing about these for me (and maybe nice for you) is they they get loud and clean then overdrive; they don't saturate at low volume levels at all. So if you're going to be carrying this thing to band practice and need to be louder than a bassist and drums, the Fender amps are worth playing around with.
Other small amps worth considering if he has one:
The Vox Brian May amp. It's solid state, but it sounds so good, and they haven't made them for a few years, so they're getting rare.

Attribution: Image from masked elwood. It's his amp.
Expensive and rare, but the various Matamp mini-amps are worth looking at.
If you don't mind a head (ie, you can manage to have a speaker cab at wherever you're taking the amp), look at the Orange Tiny Terror, Vox Night train, etc.
A little bit bigger and quite a bit heavier than the others, but a Vox AC15 is a great amp, too.
Hope this helps!