Hypokalemia is a common cause of death
in eating disorders. SOB is not.
Irregular heart rate will effect perfusion, and with perfusion altered, respiration will be effected.
But, dollars to donuts, the potassium (while not horribly low) is worth replacement- and not knowing how often or how severe the eating disorder behavior is going on (or the specifics- diuretics, laxatives, vomiting, excessive exercise, starvation - all or some of those) doesn't help- but the fainting also suggest cardiac arrhythmia
in someone who has not been noted to have cardiac abnormalities.
She can't breath because she's trashed her potassium, and has an irregular heart rate, activity intolerance, and causing fainting. Another purge could leave her dead on the bathroom floor. No joke.
You MIGHT hear irregular heart rates- but it's more likely to show up on an EKG that is ongoing for a bit. Hypokalemia has a specific EKG pattern.
Respiratory: she has symptoms, but not caused by a pulmonary problem. Address it as part of the cardiac issues.
Neuro- it's possible for eating disorder patients to have abnormal EEGs and brain shrinkage (severe cases) but with the only semi-neuro symptom being fainting, I'd still go cardiac.