Rick
First of all, a couple of useful links:
http://www.beerintheevening.com/
http://www.camra.org.uk/
Where will your friend be staying?
Central London has some very fine drinking establishments. The Star in Belgravia, the Lamb and Flag in Covent Garden, the Princess Louise in Holborn, Doctor Johnson's old local, Ye Olde Cheshire Cheese in Fleet Street. Full of bloody Americans though
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Seriously though, all of these are famous but touristy. It doesn't make them bad pubs (far from it) but it's a fact that most central London pubs won't be full of locals as there aren't many people living in central London.
If he/she is willing to travel just a little way i.e a shortish tube ride, there is any number of decent 'locals'. In particular, anywhere along the river like Hammersmith, Chiswick, Putney, Barnes, Richmond have some tremendous waterside pubs.
In central London, if they want to avoid the West End they should head for the City. The City is the square mile of the financial district. Very few locals for reasons mentioned earlier, but plenty of tremendous pubs, many ancient and historic.
When are they coming over? I could possibly be persuaded to meet up with them and give them a helping hand -- purely in the interests of improving international relations of course....