They can grow back! I think it just takes a while. Their eye can grow back too if taken, though hopefull that won't happen!
Maybe you should put them in their own home, or if your betta is in a big tank, divide the tank so each has it's own space.
Watch out for snail eggs, mystery (apple) snails will reproduce and you'll have quite a few on your hands!
Which is awesome, but may be overwhelming. Haha
Mystery snails need 2.5 gallons of space per snail. I have one and he's in a divided tank with my female, they didn't get along. lol
Keep an airstone with the snails so they get oxygen.
If you go to AppleSnail.net, you will find tons of great info on caring for your snails!
"Life is an adventure full of beautiful things, and a treasure worth more than gold."