Hi, I am thinking about asking a professor for a letter of recommendation. While I have never took a class with this teacher, I work often with her in a job setting (school tutoring). Is this fair game, or do I have to had taken a class with her?
You could use her as a supplemental LoR, but when colleges ask for 2 teacher recommendations, they mean, even if not explicitly stated, teachers who have taught you in an academic class.
Agreed. The teacher can be an extra LOR much like a person could get a coach, a priest etc. to write one extra letter if the person is able to say something about you that won’t be covered in other letters. However, it absolutely should not replace a letter from an academic teacher.
@StaticRoar: Of course it’s “legal,” you won’t be arrested for doing so, but it’s not appropriate. 
Proper use of English is important (consider your essays). The advice received from happy1 and skieurope is certainly correct.