The way that my school has it set up is that an applicant is required to submit 3 recomendations (2 science, 1 non-science) to the committe...and then if the applicant wishes he/she can submit a 4th letter. I asked the doc I shadowed for a rec as my 4th letter.
I would advise to only ask the doc if you've spent significant time shadowing him/her and if both you and the doc are comfortable with the LOR. Just make sure you've spent enough time, asked all the questions, and generally taken something important away from shadowing so that your doc has something meaningful to say in the rec.
In your case, I'd submit both a rec from your employer and one from your doc. I think both would serve as character letters, and its important to have both. The letter from your employer conveys what you've been doing with your time off from school and your letter from the doc shows your interest in medicine. I think both are necessary.