LuvAnimalsDearly -
I completely understand where you're coming from. It is always tempting when I go into pet stores to want to try to "save them all". The animals often look so uncared for and sad, and I want to rescue them. It is heartbreaking to just leave them in that situation.
What I would do is contact your local ASPCA or other organization and alert them to the sick animal. Maybe take some photos if you can do it in a subtle way. Right now the issue is a bad pet store. If they are left alone, they will keep hurting animals. If you save one animal, you'll be showing them that "hey even if we have sick animals, people will buy them, so we don't have to change. This is great!" Instead, take action so that they STOP having animals treated like that.