I totally agree…a garter stitch scarf is a good start…it is fast and produces immediate results…I would do something that has a bit larger needle, like maybe an 8…so many lessons would be learned:
cast on and off
stitching in the correct stitch (I often missed when I was learning!)
I am betting that if she is eager, she will be onto purling quickly.
I would teach continental because it is faster. I learned English knitting and it took me a while before I could undo years of doing English. Start her on the faster method. Plus, I am left handed so that is a whole other thing.