What level is your B-spec driver at? I'm guessing that he'd be around LVL6-8 given that you are doing the Amateur level.
That car is waaaay too much power for a B-Spec at that level. B-Spec's don't seem to cope well with oversteer and massively overreact. So either set up your car's suspension so that it will understeer, which is probably impossible with a RWD car with that much power or switch to a FWD or 4wd which has a more natural understeer balance.