I think that if you introduce the betta into the snail's territory, it will be fine.
And I know how you feel. The pet store near me used to take awful care of their bettas, and most of the ones I got from there mysteriously died within a month or two. Now they have much better stock, and they live much longer.
Like iloveenegl said, you have no idea what conditions she came from, so she could have had something wrong with her for a while. Maybe she came from an unhealthy line, or maybe she was raised in a bad environment.
In any case, I'm sorry for your loss. :(
And now these three remain: faith, hope and love. But the greatest of these is love. - 1 Corinthians 13:13