This is a tough question to answer. Since she's so young, there are a lot of things to consider. How overweight is she? At this age, she is growing and her body is going through a lot of changes, and I don't think she should be restricting herself from eating healthy carbs. Maybe she could start in Phase 2 instead of the more restrictive Phase 1 if she really wants to follow SBD. Possibly the best course of action for her is to just eliminate sugar, junk food, sodas, etc and stick with healthy food choices, and to increase her activity (walking is an excellent idea!).
These are just my opinions, but I'm no expert. I think the best thing for her to do if she really does need to lose weight is to visit with her doctor about a healthy way to do that without impacting her growing and changing body.