I believe this concerns the fish in this topic, with pics;
http/www.fishforums.net/index.php?showtopic=285523&hl=
By those pics there may be one of a few things, or perhaps a combination. Epsom salt will generally relieve constipation. For tough cases you can add a little to food. If this doesn't do the trick, consider bacterial problems or eggbound.
You can treat both of these at once, for eggbound fish I go with a long term fast, 4 weeks, with the hope that the eggs will be absorbed by the fish being in starvation. Couple this with Maracyn & Maracyn II. Ignore the directions on the package, 50% water change daily & remed, for at least 10 days, even if the fish seems to be doing better. Fast this entire time, after 10 days start on a very light low protein diet, small amounts of food.
This is what I do for cichlids with this problem. While bettas are not cichlids, trying something is better than trying nothing.
It amazes me that people can go over 40 posts on about a dozen fish poorly kept in a shop, but you can get only one response between two topic on a couple of ill fish, that are owned by a member as pets. More concern is shown for fish in a shop that no member is personally attached to yet, but the ones that a member may well have a personal attachment to are ignored. What sort of sense does that make?
The information on how to help these fish may be of use to any rescued fish kept in poor conditions. Without having to wait two days for information you might have the answer on what to do, how long it will take, and what results to expect. You are perhaps helping yourself in the future by helping others now.