Just one tip though, if you boil the plants, the hot, hot water make cause them to "curl" on themselves. They'll still be plenty usable, but they may not be quite as tall and floaty as before. :)
If you're worried about it, rinse them in hot water, and then lay them in the bright sun for several days, the UV light will kill almost anything.
As for the gravel, I'd either take it all out and boil it, or let it air dry *completely*. It's probably overkill, but I had some nasty columnaris many years ago when I did my first bout of Betta's, and I finally ended up realizing it was in the gravel. Once I got rid of that, everything got better and I stopped having the issue.
Probably overkill, but I'd rather be safe. :)
Sorry about your little guy though! It is amazing how fast you can get attached to them. They've got so much personality. :(