I don't think it'd be an issue. I have an marine snail shell from my old collection and I've seen another member use a clam or mussel shell as a feeding dish for African Dwarf Frogs. Just soak it in warm treated water just to make sure and it should be okay. If its a snail type shell put the opening towards the bottom so that your fish can't get stuck in it and make sure it doesn't have sharp edges (like some conches do).
As long as you're absolutely sure the fish cannot become trapped or scraped, I wouldn't be too worried... But Star Lollipop is correct that it could raise your pH. Bettas prefer to have a lower pH, so raising it usually isn't good for them.