Honestly this is a burn all bridges method and probably a bad idea but you could publicly shame them. Take pictures of the dog on the lead and put it all over Facebook Instagram whatever social media they use so all of their family, friends, & coworkers know how they're treating their animals. He probably shouldn't do this if he wants any relationship with his family but his family sounds toxic so does he?
A lesser version of this is just to bring it up when they have friends over, at the grocery store, Thanksgiving dinner, etc. In a nice tone like; Have you been working with Leia on her crate training or she still on the lead 24/7 because its so sad to see her like that? I know people who are willing to take her and give her a better life if it's still not working out.
If they're not doing anything wrong they shouldn't be embarrassed right?