Ha! You came to the right place.. Thank yourself for likely saving the life of this animal.
Feeding plant eating reptiles lettuce is typically a bad idea.. but it never fails that people are always recommending it!
Lettuce is basically water with a tiny bit of chlorophyl in it.. Sulcatas should really not be eating it as a staple food. It barely meets vitamin and nutrient levels.. and if fed exclusively will DEFINITELY lead to a slow deformative death. There's no doubt about this.
These little tanks were made to live in burrows in an area with little regular rainfall. Most of their diet in the wild is grasses and small plants that grow in the short time of year when its not too dry.
Because they come from a dry place nature designed them to seek out plants with a high water composition..
In captivity food and water are not in short supply but the animal will still always go for the high moisture items.. too much in fact.. and will make itself visibly sick with diarrhea, and in other not obvious visible. If you offer any fruit only give it a part no larger than the size of one of the tortoises back legs.. and only one time a month a most.. otherwise bad things will happen.
Feed them pesticide free grasses, hay, dandelion weeds, hibiscus flowers and leaves and other high fiber PLANTS only. Anything other than plants will make them sick on the inside.. you may not see it immediately but it will be hurting.
The list of foods they should be eating is available all over the net.. I just listed the first few I could think of.
Also natural sunlight is critical for proper bone and shell growth.. the lack of this plus a diet of good ole' fashioned lettuce is likely the cause of its (likely) painful deformity.
**What you can do:
Figure out a way to get this animal at least 5 hours of natural sunlight per day (not thru a window- glass blocks the exact types of light they need to get from the sun).
Makes sure it can get into the shade to cool off anytime it wants- if not it will die within minutes- make sure to keep in mind the movement of the sun and shadows.
Feed it pesticide free grasses and flowers.. these guys graze all day just like cows.. feed virtually nothing else.. no dog/cat food or whatever.. it messes them up inside.
Keep hydrated. Give the animal a supervised bath or soak in a *shallow* pan or pot for 15-20 min once a week.. they can't swim so make sure it doesn't have to hold its head way up to breath.. otherwise .. it will get tired and drown.
Keep the tortoise warm.. don't let the temp where he is staying go below 74F (you'll have to convert for Celsius). Remember these guys dig a burrow for shelter from high heat or cool temps. If it gets too cold.. it gets a runny nose looking thing and basically begins it's virtual deathspiral. Get it to a reptile vet if this is happening already (likely).
I hope this was helpful, everything here was off the top of my head but the info is good as I'm sure other people will agree.
A good place to get ALL the SPECIFIC info you'll need is
http://www.sulcata-station.org/
Follow the care recommendations and likely the little guy will do great. also you might want to print out some info from there to share with the others that will be caring for the tortoise in the future.
Sulcatas have great personalities and are darn tough.. you can do it.. also.. keep us posted on him and come on back with any other questions.
Good Luck
--jeff