They thought it would be easier in London on the grounds that once you're in the UK (a relatively difficult country to get into), getting a Schengen visa would be relatively easy compared with their home country of Vietnam.
Friend is coming from Serbia, the problem is that visa lines are long in the home country (actually have to queue) themselves, it may be a deterent from him for visiting other countries.
I do recall that France did give out Schengen visas for Schengen zone countries before I had a "hassle-free" passport.
Thanks for the information!