Coaster brakes might not have appropriate stopping power for your situation. They can also fail silently which I didn't know as a kid, but judging from my parents' rules about where I could ride, they were very aware of it. Make sure your friend is aware of these dangers.

Also, show her something like the various state cyclist manuals. The ones based on Effective Cycling really helped me understand braking better. When I went out to apply what I'd learned, I found I was handling the bike better. Spent a couple weeks boggling that no one had taught me to downshift for stops before. And until I read the manual, I'd had no idea that one brake had more stopping power than the other. I probably should reread and get in more practice time on other cool tricks.

If it seems like she'd like biking, any kind of bike that gets her out there and trying new things is good. Well, within reason. Friends don't let friends buy 45 lb bikes