Probably not, a serving of lettuce only has 15 or so calories in it, (dressing has significantly more....so watch that if you're worried about extra calories), but I think that eating only lettuce would be pretty unhealthy :/ It has so few calories because it's mostly water and air, and doesn't really have any nutritional value.
I'm vegetarian so I eat lots of salads and stuff. If you keep your salads full of other veggies and a few fruits here and there (try apples in it!) you should be alright.
Remember not to have too few of calories in a day, be safe!
Yeah, that would be a cause for concern. It depends on her build, but generally, teenage girls should be at least 115 or in the 20's.