I didn't know you could gain that much weight/percentage wise even if you tried in that short period of time.
She did not eat caesar salads.
I don't know what she ate, but I don't think if she were trying to bulk up with extreme high calorie foods she'd have an easy time of packing on nearly a 20% increase in total body weight in 3 weeks.
If you google "gain 15 pounds in 3 weeks", you'll find that people say it can't be done.
I'm really MORE stunned that your husband put on 20 pounds in that 3 weeks.
I wouldn't let her go with them for an extended period of time again.
Best wishes getting that extra weight off her and retraining her body.
It sounds like she didn't get any exercise. Sometimes when kids are with adults who don't have children, all the adults can think of to do with the kids is let them watch TV or do things on their computer, and if a kid sits around all day and eats restaurant-sized servings of food, she is going to gain weight.
Can you and she join a parent-child gym or swim class and get moving? This is not something she can solve by herself, and she would certainly appreciate your support. It will feel much less like judgement if you do it with her.