You can't teach someone "how to compose" because you're teaching them how to shoot as you would shoot. Instead, what you're aiming for is teaching them all the possibilities... about working the image to begin with. For example, the biggest mistake beginners make is to just point the camera and shoot. This means everything shot is from the same position and angle... No matter what they're shooting. Get them to slow down, and actually LOOK around the viewfinder and consider other ways to shot other than the first initial instinctive way.
Demonstrate the "rules" but make it quite clear that obeying them rigidly will produce rigidly boring images. Show examples of why sometimes it's best to break the rules. Above all though.... get them into a culture/mindset of not just pointing and shooting. Get them to spend some time walking around looking through the viewfinder from different angles, heights etc. Then make the choice about when to press the shutter. It will also embed a culture of NOT machine gunning and chimping.
Others have mention this too, but working in the above way is also a great way to evaluate other factors that change with position, such as light and tonality. If you just shoot what's in front of you without viewing through the viewfinder from all possible angles, you may well walk past a great opportunity and never realise.
Don't get too wrapped up in composition though... it's something that is part of someone's style. It grows and develops as they do, so try not to be too prescriptive at the early stages. Post examples of Alec Soth's documentary portraits in here, and most will say they're badly composed because they're bang in the middle of the frame, but that advise, in his case, would be utter [PLEASE DON'T TRY TO BYPASS THE SWEAR FILTER].
To recap: Get her to view the image (through the viewfinder) from every conceivable direction, height, and angle before allowing her to press the shutter. That's my advice. She will then develop her own style... and when she does, try to resist critiquing stuff negatively because it's not how YOU would have done it. She's not you

[edit]... obviously, distracting backgrounds and cropping off hands/feet without good reason is something that needs addressing.
To answer your other question. I'd be tempted to teach this later when she can actually operate the camera well enough to get correct exposure and focus. You can make comments, but let get her head around metering and focusing first.