It would be nice to know whether this setup (i.e. the position of the gastropod foot and mantle) was in situ found or just the result of storage in ethanol. Usually, when olive snails are active, their mantle and foot extensions partly cover the outside of the shell. One would think it makes very difficult to get anything between the mantle and the wall of the shell (even much smaller things than a hermit crab). So maybe due to the injury, the snail was partly withdrawn into its shell when the crab invaded its shell.
Another thing: for non-specialists it would be useful to indicate the injury on the snail's foot, and also to indicate which is the anterior and the posterior part of the snail