given that snails are not known for eating anything whole, I find the story highly questionable at least. It is possible that a 1" turtle ended up inside the shell of a 7" snail if in fact they were really those sizes and really in the same tank. Inside the shell and inside the snail are two drastically different scenario's especially from this particular turtle's perspective. I cannot see a turtle holding still while a big snail rasps it to death, and I cannot see a snail eating a turtle whole either. If you've ever skinned a turtle (I have) you'll find that they are as tough as boot leather everywhere that isn't protected by the shell. Even a baby turtle is pretty tough to be eaten by a snail.
And of course as was mentioned, I can't see a 7" snail hitting the wall hard enough to break and not hurting the drywall either. The story is not impossible but seems quite unlikely to me.
Dave