If your girl isn't afraid to take some lumps then the best place to learn is a blue slope with a mixture of slightly steep slopes (to force her to learn to maintain an edge to brake) and moderate slopes so that she can get her slide on. 6-inch powder is ideal because it will offer good traction and not be too ridiculously difficult to stand back up in when she falls.
A green slope might work, but in my experience it can difficult to find greens that are steep enough and haven't been plowed to infinity by lunchtime.
Either way, just be thankful that you aren't this poor bastard that I saw while I was riding the snowflake lift at Breck.
They were on Four O'Clock run, a green. Notice that the girl is either too afraid or too uncoordinated to get herself going down the slope with enough speed to actually LEARN how to control the snowboard. I actually watched this guy unstrap his board so that he could push/walk her down part of the slope. That guy in the background was his (obviously bored) friend.
I'm not an instructor, but IMO it would be far more humane (and productive) to find a steeper slope and let her figure use gravity things out after showing her how to do a falling leaf.
Try not to be this guy!