This answer is part of Pet's Spring Cleaning Campaign. This question is old, but this answer will still help people with the same problem.
The short answer is that yes, the snails probably are suffering, but we don't know if this tank is their usual enclosure or just their temporary home for a week.
The size of the enclosure is not even enough to house 2 snails, not to mention 11 of them.
The Royal Society for the Prevention of Cruelty to Animals (RSPCA) writes about the care of Giant African land snails:
Two land snails will need an enclosure measuring 60cm long x 45cm wide x 40cm high. More snails will need a larger tank.The enclosure should have 3-6cm layer of peat substitute on the floor that needs to be kept damp but not soaking.
That there is soil in the tank is good and the snails like to bury into it, but the soil should be kept slightly damp.
A water bowl should be placed in the enclosure for the snails to drink from and to help keep the humidity at the right level.The tank should also be misted lightly with warm water every other day to maintain the moderate level of humidity.
As to the diet, lettuce alone is not nutritional at all and the snails need a much more varied diet.
Leafy greens including lettuce, cabbage, spinach, watercress, land cress, dandelions and grass should all be on the land snail’s menu. They will also eat vegetables and fruit such as courgettes, cucumber, apples, melons, grapes and bananas.
Calcium is also an important part of a land snail’s diet for normal formation and growth of the snail’s shell. Calcium can be provided by giving the snail a cuttlefish bone, natural chalk, crushed eggshells or as a powdered supplement sprinkled on the food.
So if those snails lived in those conditions for a long time, they would most likely suffer. There's a chance that the owner does have a bigger enclosure at home and feeds them a more varied diet, though.
According to Petsnails.co.uk, the signs of illness and stress (possibly caused by an unfitting environment):
- The snails spend much time buried in soil and are generally inactive
- The snails retract deeply into their shells without sealing the opening with a thin membrane
- The snails body looks too small for the shell. Both shell and body lose color and look bleached
And if the diet contains too little calcium, the shell will get lumps, bumps or small white patches and might even break.