One thing Tiari doesn't really mention (and which may not apply in this particular case). Bettas tend to be bought by newbies. Newbies make mistakes. With anything below 5 gallon you have NO room for error so any of the common mistakes like replacing all the filter media at once can quickly be fatal.

I've also got an account on yahoo answers which is littered with teens and their bettas. I tend to post this as a standard answer:

Keeping a betta properly.
1. get a decent sized tank with heater and filter. 5 gallon at the very least. Despite the fact that they are kept in cups in the shop doesn't mean they don't like some room to swim. A 5 gallon or better will also provide some room for maintenance mistakes. Filters, look for something mild current, you don't need a massive filter. Sponge filters would work.
Also get a bottle of a good water conditioner that handles chlorine and chloramine. No untreated water should ever go into that tank. Salt is also unnecessary, it does not do anything for a freshwater fish.
2. Either cycle it properly (see below) or get a bottle of Tetra Safestart and used that together with some flakes for a week. Do a 60% water change after that before adding fish.
3. Bettas really like some cover. Real plants, silk fake plants if you must. Some like a cave. Don't use plastic plants they can cut their fins on it.
4. Feed as much as a betta eats in 30 seconds twice a day if they're still growing. Once a day and 60-90 seconds if they're not. Use a good staple like this
An adult betta can fast one day a week without problems and will love the occasional treat of thawed bloodworm (3-4 at a time)
5. Bettas do not get "lonely" but if you like you can introduce a few small ornamental snails or, in the bigger tanks, a few shrimp.
6. Do NOT EVER EVER EVER replace all the filter media at once. You lose your beneficial bacteria and reset the tank to new.
7. The only time a betta will tolerate another betta is when they are ready to mate. At all other times a male betta will kill any other betta. It is possible to set up a sorority with a few females but you need a 20 gallon at least with plenty of plant cover.

Cycling information:

I hope this has saved a few fish. (if anyone has suggestions to improve that 7 step list they're most welcome!)