Hi
Breathlessness on exercising could be due to a lung problem or due to a heart problem. Lung problems like asthma, emphysema, bronchitis. Heart problems like valve prolapsed, cardiomyopathies etc. It could also be due to diabetes, anemia, obesity or a thyroid problem. All these problems can start in early teens. I think your friend should discuss these possibilities with her doctor. She may need to undergo a few tests like blood tests and ultrasound to reach a diagnosis. I think you should discuss the possibilities with your doctor. Please let me know if there is any thing else and do keep me posted.
Communication is key - her doctor needs to know that the allergy pills are not helping her symptoms. I don't know if her being a teen means she is a minor, but if so, hopefully she is communicating her continued problems to her parents and they are taking them seriously. Any unusual breathing difficulties merit a good look to rule out medical problems that need treatment.