What is her personality? Would she be more inclined to want, say, magenta walls or more of a neutral, like a grey?

She wants classic. Any exuberance will be in some pops of color in pillows or accessories. She just DIY -lovely headboard covered in navy linen.

You can go with a soft beige for warmth

[Eclectic Bedroom[(https://www.houzz.com/photos/eclectic-bedroom-ideas-phbr1-bp~t_715~s_2104) by New York Interior Designers & Decorators amanda nisbet

Or you can think about a very soft blue wall.

[Midcentury Living Room[(https://www.houzz.com/photos/midcentury-modern-living-room-ideas-phbr1-bp~t_718~s_2115) by San Francisco Interior Designers & Decorators Niche Interiors

I think I'd go with a mid-gray, i.e.: not too dark and not too light, to give the room a serene, stormy, ocean feel, then use fabrics (bed, chair) to add a little warmth. Maybe deep browns. I have this rug in my bedroom and absolutely love it. Thank goodness I saw it first in person - and the second I saw it, I knew it was "the one" - because the on-line photo is actually awful. But it's got soft browns, grays, blues - I thought it was called "Nantucket" somethingorother, which would make sense. It's actually a Hampton Flagstone. It goes with everything, but adds a softness to the room. It's a wool blend, and I have a good pad underneath it so it's comfortable underfoot. Also, it has fringe on one end, which I hated, so they sewed a brown border on it for me, all four sides, like sisal rugs have.

The gray and navy and a rug like this with would give the room a beautiful sophistication, I think.

I like the soft beige example above. I did a quick houzz search, maybe she'd like to look at the visuals and see what suits her.

Here is a link that might be useful: Navy and white bedrooms on Houzz

Gray and navy

Navy headboard, gray wall, although I'd pick a grayer gray and a bit deeper. Warm rug.

Or maybe a mocha/brownish gray? I used all Ben Moore on my walls, but only because I'd forgotten about C2 paints, which come in amazing colors, and you don't have 8 million to choose from like with BM and SW, etc.

Check out, for example, "Smidgeon" Open the link, and in the box on the bottom right-hand corner ,type in 912, then click on the teeny box in the upper left hand corner of the bigger box to see the color. With navy against it? Gorgeous! I'm going to repaint using all C2 paints.

Here is a link that might be useful: C2 Smidgeon #912

I think the pale gray works nicely, but I can tell you I live with navy upholstery and shaker beige walls and I love it.

This is a completely different direction but I wanted to share that my 17 yo's bedroom has mint walls and navy and white bedding and accessories. It's very fresh looking.

a warm beige softens it enough to say sweet enough and not sugar-sugar.

I'd think about a dramatic navy, just different enough in shade than the headboard, that the headboard becomes a texture. This is probably wallpaper, but the depth of color I was thinking.

or

I have a soft yellow painted room with navy chairs and couch....really like it.

Why not white?

If I were going to go with a color, the pink or green factor would make a significant difference for me. For a neutral, I'd go more grey with pink and more beige with the green, but a warm grey or softened beige would probably go well with them all. I have BM Wish in my bedroom with a pale blue ceiling and white trim. It goes well with all shades of blue and most other colors.