I'd just make him wait. Still help him, but not jump on it. That'll give him time to reflect on buying beaters and will make it less attractive in the long run. I am sue you have perfect excuses why your wife needs you at home and that you'll help him later. Your shop is all his, if he wants to tackle it himself.
Seriously, if he doesn't wrench himself, he has no business buying old heaps.
G