Since you are happy with everyting but the B, don't change anything but that string. Search around for single B strings and start a systematic comparasion of the different strings. Leave things like Bridges, preamps, tone controls, etc.etc. until the very last.
And if the problem is the setup or technique he can end up going through strings and never addressing the real issue. Whenever I tried to tackle a problem like this, the first thing I looked at was me.