Hi,
I haven't tried this myself as I'm a relative newbie to tropical fish, but I've rescued various creatures over the years and this is what I'd do (maybe someone could try it and tell me if it's successful):
Firstly, no matter how disgusting their water, it's water they are acclimatised to, so put them in a small bowl with all of the shop water and some extra, conditioned water at the suitable temperature (try to avoid them getting chilled in the journey). Keep the water shallow initially, as they will not be in a fit state to swim to the surface.
Don't use any tank furniture or gravel in the initial stages, except maybe a plastic plant, but put black paper around the outside to reduce stress on the fish. Add an air-stone that produces very fine bubbles (bettas hate currents and large bubbles) and keep the temperature constant (no more than a couple of degrees higher than in the shop). I believe a slightly higher temp will produce a higher metabolic rate in the fish and aid healing, but that's just my personal theory.
Treat the water with Melafix, a tea-tree based natural remedy which is good for fungus, bacteria and viruses and boosts the immune system. Don't treat for anything else unless the fish has symptoms of frank disease (and even then, give the Melafix a couple of days to do it's magic first).
Keep the betta as quiet as possible (the small bowl or tank will reduce swimming about which might tire him) and feed on the best invertebrate food-stuff you can get - live brine-shrimp, frozen bloodworms etc. If he's very sick, you might have to make sure it's dead food, since he may not be able to catch it.
A vitamin supplement such as eSha's "Optima" may help.
Then change no more than 20% of the water each day, being careful to keep the pH and temperature steady so as not to shock him. If he recovers, you could gradually add a little more water each time until he is in the normal depth of water. Only when he is fit and fins are growing back should you even consider a proper betta tank or a community tank.
I'd value comments on my cunning plan, for any of you.
:love: