I would tell them to think of warmth... and go from the cold of the blues and purples all the way up to the red hots.

I would try associating taste with it.
Example Orange for orange, Barley health drink for green, blue raspberry Popsicle for blue, lemon head candy for yellow, smell of lavender or lavender water for purple, cherry or fire balls for red.
First and foremost it would help to know if they are an auditory learner or tactical. Good luck most of all. I find for myself people learn more if its fun regardless of their age.

Try to explain it like a sound. Blind people are more sensible to sounds.

I'd try to relate it to on of the senses she does have. There's no way she will actually experience a color, but she could understand it metaphorically.

You could set up a vibration table and have her put her hand on it. Then gradually increase the frequency until she can track how fast it's going, then increase it until she can't tell how fast it's going any more. That range is kind of like the visual spectrum. There is more "light" that vibrates faster and slower, but the visual spectrum is the range we can deal with.

You could do the same exercise with a speaker, by increasing the frequency of the sound through the range she can hear and then above it.

Blind people have learned how to "see" the world with echo location. It's teachable. You could help her learn that and then she'd be able to "see" at least a monochromatic image of the world. Then she could imagine that some things have different "vibrations" instead of colors.

I would approach it through a sense she does have, perhaps touch or sound. Colors have different "feelings" just like touches do: red is warm, like maybe a warm metal pan; yellow is warm and light like a warm delicate curtain or baby blanket. Also, green things have a lot in common since most plants are green. And in explaining the rainbow, you could demonstrate how one color blends into the next. Good luck, I'm sure your friend will enjoy the process.

use taste to help. Hot is like red. peppermint is like white or blue. raw spinach taste like green. color is just a fragment of white. use temperature and taste to demonstrate how color varies too! A rainbow is like a dish where you can taste a lot of individual ingrediants.