Did my betta die of Neon Tetra Disease?

My betta, Memo, died yesterday. A couple of days ago he ate a dead Neon Tetra I bought on the 20th. His skin turned whiteish and his tail and fins fell off, they almost looked ripped off. Him and another Tetra where the only ones in the tank, so do you think it was NTD? I want to warn petsupermarket that they sold me sick fish but if I do that their just gonna kill all the Tetras and thats really sad, so I'm gonna keep the Tetra until it dies. I'm just curious, his death was so sudden it really made me depressed.