Vision is one of our five amazing senses (sight, taste, touch, smell, hearing). Each sense helps us experience our world. People with the inability to see must rely on their other senses to help them understand and "view" the world around us.
A rainbow is a meteorological phenomenon made up of six colors: red, orange, yellow, green, blue and violet. To help a vision-impaired person understand a rainbow, I would emphasize the senses that he or she is capable of experiencing. Blind people may not be able to see differences in colors, but they can understand concepts such as hot and cold and hard and soft; colors have similar characteristics. If I had a musical instrument available (like a piano), I would play six notes with each note representing a color of the rainbow. The differences between the musical notes might help your friend imagine intensity in the colors of a rainbow.
I could also create a rainbow on a piece of cardboard, making each color of the rainbow out of a different texture (red could be created from macaroni, orange could be created from oatmeal and so on). The shape of the rainbow pattern on the paper and the texture of each color in the rainbow might give the blind person a better understanding that each color in the rainbow is different from the other. You could also use the sense of taste or smell to create a similar contrasting experience.
A blind person may never see a rainbow, but they still have the capability of creating a picture in their mind of what a rainbow might look like.