I'd be inclined to tell them why you'll be staying away: it's likely they need the business & if they clean up their act, it's a "win" for their animals. Otherwise,
consider how many others will buy from them without knowing any better, & ultimately it's the animals that will suffer. Some pet store owners/managers are
knowledgeable, others not so much: it's possible they haven't noticed the issues, & maybe they're coming from their supplier. They might even appreciate a
"heads-up"?
I'm also wondering if they're in violation of any laws...I think I'd also call animal code enforcement for the city, just to see if there's anything they can do?