There is one male crowntail betta in a 1/4 gallon jar. He doesn't belong to me, but he belongs to one of my mother's co-workers.
His current state is very dire - there is a black coloration growing up over his body, his body is expanding rapidly far beyond its normal size. The worst part is that there is a red turmor-like growth coming from the area where he excretes waste. It looks like he's hemmoraging his guts out. White frothy fugus is growing on this "tumor" and the betta is having a hard time swimming and continually bobs up in down because he can't stay level.
I've treated him with the proper amount of salt and QuickCure, which has malachite green. We're changing his water twice a day and I've advised the woman not to feed him, because we don't know how this might have happened.
His previous behavior was that he was listless, sat on the bottom of his bowl, and would not eat. Please, guys, I need to tell this lady something. She's desperate for news to help Blue. Thank you so much.

~ Repsychus ~

Oh, on another note, this happened within a span of 48 hours. I don't know any sort of disease that could have caused that. Please, help us!

Well a 1/4 gallon is very small so my imediate thought was that the black coloration is ammonia burn. The bloating may be dropsy, related to some kind of internal infection. As for the 'tumor' it sounds like maybe some kind of prolapse..but I've never encountered anything like that in fish so you'd be better to wait for someone more knowlegable with bettas for help with that.

I think the treatment you've given so far is good, with the salt and witholding food. I'd suggest moving him into a larger container as well, even if it's just a big food storage container or something. Hopefully someone else can offer more help. I hope the little guy comes around

How is the little guy doing?

I think Chrissy Bee is right. It sounds like dropsy. I would get him in a bigger container as well .Maybe something that could be heated. That may have contributed to his illness as well- poor water quality and low water temps.If it is dropsy he may need an antibiotic. Dropsy moves quickly so hopefully you can save him