1200 kcal for a teenager after cancer treatment seems to be low to me as well. caloric requirement depends on weight, age, gender and particularly medical conditions and treatment. i'm sorry that your daughter doesn't seem to like the recommendations by her RD

if possible, ask your doctor for another referral to a dietitian who has oncology experience.
in my opinion, in order to create a positive attitude towards eating in children and teens, i don't like to name a food "bad" or "good". a dietitian colleague of ours Ellyn Satter is an internationally recognized authority on eating and feeding and has spoken in thousands of conference in the past 20 years about this topic. you may check out their work here
http://www.ellynsatter.com/