The key is stories and play. That's how the brain works (even for us "grown-ups"). I associate stories with the names of the notes on the piano (the Dog lives in the "Doghouse" formed by two black walls, the cat is off to the side trying to get in, and the elephant stands guard on the other side. And so on and so forth (the Cercone Method uses such mnemonics to teach 3 - 5 year olds).
When teaching younger students (and, by the way, I usually don't go younger than 8, although I have in the past), I use their ear for high and low notes, stories to name the keys on the piano, and fingering to help them play from set hand positions (and, yes, I write the finger numbers on their fingernails with wax pencil. Hey, it works!). Kids love to play on the piano, and I try to make everything as fun as possible. Good luck to your friend!