There's a few points to consider here. Firstly, if he'd been playing for so long why are his core muscles considered weak? Surely after all this time they should be strong because he played four times a week on a regular basis. So, either he doesnt need these muscles to play (he was pretty much injury-free) or his technique was questionable because he wasn't using his core muscles in the right way according to those advocating core stability etc.
If he's not playing 'properly' then would a course of exercises that have nothing in common with natural golfing movements really help? Would he suddenly have better technique after doing them? Or is it the case that, actually, core strength isnt really an issue for golfers?
My belief is that we should not focus on individual or small groups of muscles and work them in an isolated fashion as it doesnt promote natural coordination. A golfer should focus on how they move, that is, their technique, and if you move well, all your muscles will be conditioned in a way that is relevant to golf. Exercises to develop specific muscles do not teach you how to swing, putt or chip.
Your thoughts are welcome :0)
For more 'out of the box' thinking on improving your game see my book - Golf Sense.