In a practical sense there is no role for level testing in your friend's case. Since the Pill is being written for symptom improvement, if the symptom is improved to her satisfaction the dosage is "effective" or "correct", regardless of what her serum level might be. Similarly, if she is having side effects she doesn't like, her dosage is "too high", again, regardless of her serum level.
Level testing is valuable in very specific situations such as replacing hormone levels in those with a deficiency, or in preventing a condition so detrimental that you can't tolerate a single episode, such as seizures.