I would just let him do it and when he does not get what he thinks he will get, he'll be totally turned off on gear and probably say "that shit must have been fake cause I didn't gain anything!"
Let him waste his money! Most people don't realize steroids don't get you big without proper training and diet. Those are the people that will always be average.
I don't mean to be a dick but...Tell him straight up that he is pretty ignorant and doesn't know what the fuck he's doing. Tell him the truth, that his training regime is a joke and he is completely clueless. I only say this becuase this is how I would approach a friend of mine with these circumstances who was about to take the "plunge" into AAS and it seems your sister's boyfriend needs to be approached in a very stern manner (if you will) becuase he seems not to listen otherwise. Thats just what I would do in this situation. Peace and Good Luck