I can only talk about London, cos that's where I'm from and that's the only scene I know about....

Well the really obvious place your friend really HAS to play is the Ain't Nothin But The Blues bar. It's in Soho, central London, and as it's name suggests doesn't put on anything but blues music 7 nights a week. At the very least he could go to the open mic session they do on Saturday afternoons. www.aintnothinbut.co.uk

Secondly, he should contact the Green Note cafe in Camden. They put on a lot of visiting blues, alt.country, americana, bluegrass etc bands. Additionally they also have a regular specialist blues monthly night, which might coincide with his visit. See www.greennote.co.uk

Then there's Brooks Blues Bar in Putney, south London. Never been there, but I keep meaning to. See www.brooksbluesbar.co.uk. Acoustic country blues is their bread and butter. If your friend's any good I'm sure they'd love to have him play.

Might be worth your friend trying to contact Mark Pavey, who appears to be organizing the fortnightly "Davey Graham and friends" residency at Powers Acoustic Room in Kilburn. It's a new regular night centred around Davey Graham and Wizz Jones and I read a post somewhere wherein Mark was actively seeking submissions and suggestions for guests.
A bit of searching on myspace should find you details about this night.
Failing that, I'm sure yr friend could just get a straightforward ordinary gig slot at the venue: www.powersacousticroom.com