A 2bolt booster may be enough, he might not even need one as it might just be a bleed issue.
The SL can be water bled, I think it was Mark W that used one a while ago with water and it was alright, it was only one of the first generations of tr / sl that leaked If I remember.
Long lever = more breaking power, this bring me to thew next bold statement. You've suggested that this guy used a booster and yet not your mate with the horrible, flexy frame?