I go through this situation all the time.
I share a house with 6 other people. All of which are into reptiles/or animals. But the only one who is INTO reptiles like I am, is me. The rest enjoy reptiles, and care for theirs as best as they can.
But I am constantly saying things, giving advice and one of them is constantly doing things wrong. His Cham is not being treated poorly as some but not the best either. But after months of advice a person can only say so much. I have to make the choice of being the house nag, moving out, or remaining here and keeping my friendships intact.
I believe the only thing you can do is A. Not give him any animals. B. give him advice, but know you can't change another person. Unfortunatly there are people all over the world who do these things to animals but that doesn't mean we have to never speak to them again, you know?
What I do is try to give advice but if my roomate doesn't listen, when his birthday comes around or christmas, etc I give him a gift for his Cham. I.E. A new UV tube, vitamins, a heat lamp, a certificate to the pet store for a variety of bugs, etc. I find this helps. This way I don't have to be rude or a nag, and I can keep my friendships intact while helping the animal out.
That's the best solution I have found