This isn't necessarily true. My letter writers knew me extremely well and still asked what they should emphasize. It's a normal question coming from someone who doesn't regularly write med school letters.
OP, a letter from a supervisor is fundamentally different from a letter from a teacher, so ask her to focus on those things that make it different. Ideally you want her to testify to your ability to build rapport with patients, your ability to complete projects independently, etc. The types of things that performance in a class doesn't necessarily show.
I asked a volunteer supervisor to specifically focus on 'soft skills' in her letter about me, because I knew my other letters (class and research) would praise my intellectual and technical skills, but wouldn't be a strong testimonial to my strengths in one-on-one interaction with patients.
Edit: Just reread and realized this supervisor didn't supervise you on the patient aspect. If she isn't able/willing to testify that, it does make it am much weaker letter. Saying you helped out in an office doesn't add a lot to your application. Doesn't make the letter useless, but you may want to consider other options.