Teach her to play something simple by rote, such as mary had a little lamb on the black keys?
I'm prepared with a few analogies that I remember from my really early years. There's the hand shape one, where you want to pretend there's a (small soft bunny?) underneath, and naming the keys on the piano such as D being like a Dog in his doghouse (between the two black keys) off the top of my head.
? Is that even possible?
Sure it is. On the three black keys. A# G#F# G# A#A#A# ...etc. I suspect some methods start on black keys because it is easier for students not used to curving their finger. White keys they would have to curl more.
Well, I had my first student yesterday! I only have one. She's the cleaning lady's daughter. Rumanian and very bright. She loves music. She's 10, I think. Anyway, it was her first lesson. I followed two books for beginners. At the end of the lesson, she could find c and f everywhere on the keyboard. We did a little ear training and she was able to recognize, bass notes and high treble notes. And she even recognized the difference between c and f, even if she said they sounded alike. Then she played using only three fingers. I taught her how to sit, how to hold her hand, how to play the thumb. She caught on pretty fast. It's difficult to get her to hold her hand in a round position, so I support it sometimes and remind her how to hold it. Anyway, she had a good time. She's very outgoing and talkative and I found that after about an hour she was losing concentration. Anyway, it was neat!
If you think relatively, and if you don't think note names, then black notes are the easiest of all. There are only 5 notes and you get the pentatonic scale which many simple songs contain.