Although it is possible under certain circumstances to apply for a visa in a third country, the applicant generally has to have some sort of ties to that country (for example, studying or working there). The best thing for your friend to do is to contact the US Embassy in London to explain his situation and to see if it will accept to process his application.
Edward Weiner
Attorney at Law
E-mail: E.Weiner@immigration-aux-usa.com
Internet Site:
http://www.immigration-aux-usa.com/