Everybody seems to want to convert from the BW35 auto to a manual set-up of some kind for performance or economy reasons. That's fine but if your buddy likes driving auto and can absorb a couple of mpg, fitting the $50 box is very sensible, if it's needed. Better still, if the complete B20 engine/gearbox package is available, buy that and fit it as a straight swap. Other choices mentioned start with spending several times as much and then you have LOTS of work and time to consider before the car gets back on the road.
Some parts for the BW35 are hard to find but some parts from the BW65 can be used. BW65 clutches give more solid changes at the expense of a little less smoothness. It's sometimes very minor things with the auto that get annoying, like missing bushes in the selector mechanism which make all the difference to gear selection. Adjustment of the kick down cable and correct pressure settings make a difference too.