I agree it's awful to see things like this, but truthfully we can't save them all.
I would at the very lest speak to the manager. Try to be calm and collected and state your concerns as valid points, not by arguing. You might begin by saying this animal seems unhealthy to you, and ask if he's noticed it. Inquire about feeding habits, or ask if this particular animal might possibly be housed separately for a short time.
If this falls on deaf ears, then the next step is to stop shopping that store. Sometimes your local newspaper can be persuaded to run a short column about neglect seen in a particular store.
In a last ditch, all-or-nothing, to **ll with what anyone thinks move, you can try to get enough friends together to picket the store, where you will be sure to end up on the news, and possibly in jail if you aren't careful. I have seen that happen once here, the store in question stayed in business, but no longer sells snakes or reptiles of any kind.
Gale