A tough one, but maybe they just aren‘t aware of the damage they‘re probably doing.? I remember a neighbour pointing out to us that it wasn‘t a good idea to keep throwing balls for Humphrey when he was little. Although not my first dog, he was my first puppy and I just hadn‘t given it much thought, I was grateful he‘d pointed it out.
Admittedly it depends a lot on your relationship with them and how you say it but for the sake of the pup, I think I‘d try and approach the subject diplomatically, if an opportunity arises.
Best of luck