Makes sense to me. Let us call the scrap yard and find out what they give for batteries while we look over all this clean casting lead for sale. And if that don't work stop being an enabler. Take them to scrap yard, or call scrap yard to pick them up. If he gets a little miffed, that's ok, doing dumb stuff should have negative consequences, insisting on doing dumb stuff in the face of your objections should have worse consequences.
You know when I hear about someone just bound and determined to do it the stupid way I generally like it best when they are arranging flowers stupid, or baking cupcakes stupid, or even painting their deck stupid. I just cringe when I think of someone that would ignore all the accumulated knowledge and even simple common sense from this forum playing with molten lead in preparation for playing with firearms. Sorry people who are that willfully foolish should be encouraged to take up badminton or quilting because they are a hazard to themselves and others around things that can cause injury in not handled with some care and thought.
Sorry to put it so bluntly but you need to tell him we sell them and buy lead or I'm going to call and have them disposed of. My house, my rules, even if you refuse to go along with them politely. I don't have to convince you, trying to was a courtesy, one you didn't appreciate. Now I'm telling you we haul them from here to scrap yard for the scrap value or I just call the scrap yard to come get them and they pay nothing but you don't do stupid things with them. Which is it going to be?