xV, got a QT tank? I've had mollies do this and in fact had one just collapse like that about a week ago. I got out an old 1g (that Walmart pentagonal thing; I think it's meant for bettas) and went to work to see if this one could be saved.
An airstone (don't have any spare filters), threw in some guppygrass or other random floating plant. Loaded up to medicinal levels of salt (about a tablespoon) and some old "Tetra AquaSafe" in those little packets they include free with new tanks, also about 1 tbs. And started feeding her six times a day.
and dang if she isn't making a comeback. Her belly is back to convex instead of concave, her tail muscle is filling our from paper-thin, and she's still swimming. In fact I'm thinking about putting her back in the main tank in another couple of days.
I still have no idea what the problem was. It's hard to see those worms Platytudes mentions on a black molly but I've looked as best I can and see nothing. I've cut back feeding to a couple of times a day (usual for everybody else is 1 feeding in the morning) and she is maintaining her "weight" or at least size.
Don't worry about cycling the tank as it's easy enough to scoop out a couple of glassfulls of water every day or so, and in this state excess poop (or any for that matter) is unlikely to be a problem. Re-dose with appropriate amounts of salt and whatever else you're using for the amount of water changed.
Good luck. I hope you can pull him through this.
