Hello Meg! Welcome, I see this is your first post.
While monochrome works are typically done in charcoal, graphite, and single hue pastels, I see no reason why a monochromatic painting could not work quite well and be quite dramatic!
I don't know if your subjects will be together in the same painting, but if you can get something on her person - a hair bow, a necklace, or something around the area of the head that you can add a touch of color to, that spot will immediately draw the eye (which is why you want it around the head).
Either way, it would be a wonderful way to express value and take the painting to a very finished state. Here is a
LINK to one of mine in monochrome. Now, this was taken to color, but it shows that you can get very finished with just a range of values of a single hue.
Is it traditional? No.
Is it trendy and designing artwork around decor? Yes. (We as artists may scoff, but EVERY client does it!)
But the really important question is....
Do you feel that you can produce a wonderful portrait for the client using just a monochrome palette?