Teenagers take their cues from adults. This is especially true with female teens and their mothers.
If the mother is very conscious about her weight and appearance, then the daughter, even at a very young age, will pick up on this. It is normal and fine to be concerned about personal appearance. However, when it gets to the point where the solution seems to be found in a pill, then that is cause for concern.
Healthy habits are developed over time. It seems more likely that young women, in this case, will be less inclined to engage in habits that lead to being overweight if they avoid taking diet pills.
If the young adult sees older folks in her life whom she respects and admires engaging in healthy, wholesome behaviors, then she is more likely to follow suit.
Sadly, if the young lady sees an obsession over body image, plastic surgery, dieting, etc., rather than measures taken to promote healthy living, then she is more likely to do the same thing.
If she as an adult gets to the point where with the consultation of her doctor that things like plastic surgery and diet pills are appropriate, then so be it.
However, as a teenager, she should model healthy living to develop habits to last a lifetime. The weight loss will come in time.