Hi,
I know I'm new here so I'm hoping this doesn't rock any boats
First of all I'll say good for you, for being so compassionate and caring so much about Bettas to want to do this.
However, in my personal and humble opinion I think you (i.e. people who 'rescue' animals under these circumstances, not just YOU personally) are unfortunately doing more harm than good in the long run. You see, pet shop owners/managers who will allow their stock to get into such horrid conditions really don't CARE that they're healthy and happy or not, so long as they sell. For as long as people are willing to 'rescue' them, he'll continue to buy in and abuse more to be 'rescued' all over again. In effect, your kindness is perpetuating a cycle of animal cruelty!
As was mentioned earlier, your local council's environmental health department are the team responsible for licensing pet shops. A word in the right ear about your concerns would do well. Personally, I'd forget about being a "regular customer" of such a shameful place and take my money elsewhere even if it meant waiting for deliveries! I wouldn't wish to work for such a person either, which is something to consider. Would you be able to hold your tongue once working for the man?
All the very best to you. As I said this isn't a dig or anything, just some food for thought. I sincerely hope the little guy recovers soon.
Cheers.