I have never heard of this before. I dunno what is going to happen to him. I would suggest getting some bigger snails, or not getting any snails at all. I guess time will tell if your lion will be ok. Did it eat the whole shell or just a piece? I would think, depending on the size of the shell and fish, that it would be able to digest it and be ok. However I am a human nure, not a fish nurse. haha just a little fish humor for you. I would see if he eats, and poops, and of course if his belly gets smaller in the next couple of days. If he does all of these, then I am sure he will be fine. I know that this not much help, but I dont know how many people have experienced this.
HTH
Leopard