Cut flowers depress me because they die and have to be discarded. I prefer live plants. Something like a potted daisy would be ideal.
all varieties mentioned can probably be found in silk form - much more lifelike than plastic - no aroma, no easy death - silk flowers are a good choice no matter the variety
Idlewild, you’re right, of course, but that’s taken care of, actually (sort of… she likes friggin’ pussy willow, which isn’t a flower but is her answer to the question anyway. Also morning glories. So if somebody out there likes pussy friggin’ willow and morning glories, well, what’s your third favorite?).
But I’ve given her what she knows she likes already. I’m shooting for out of the blue this time.
Carnations, live plant version in a pot, the kind that were bred for scent not looks, and the color doesn’t matter.
Gardenias and plumerias would be a good choice as well, again as a live version in a pot.
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Pink tulips mixed with white bleeding hearts.
Two things I’ve found:
The best kind of flowers seem to be the kind that you cut and arrange yourself.
Her mother or her friends will know the correct kind of flowers if you can’t figure it out.
Tall bearded irises, the lovely big pale lavender ones. My mother had a few patches of them that always bloomed right around my birthday, and I always got to cut a big birthday bouquet. Then she ripped them out to make a rock garden and THREW AWAY THE BULBS. :mad:
I’m slowly tracking down my own collection.
Red carnations. I’m cheap!
Yellow orchids, or weird and ugly ones. But not cut; still on the plant.
Hydrangeas or peonies.
I’d prefer a potted plant or something I can plant outside, but if I’m going to get cut flowers I’d want daisies or mini sunflowers.
Daisies.
If she likes pussy willow and morning glories, she might also like other wildflowers.
Either an assortment of zinnias, (varying sizes, and colors, I love most of the ones on the first page of that image search) or maybe cornflowers and corn poppies, maybe with some California poppies for color as well. Daisys are also welcome at any time, so long as there isn’t a lot of frippery with them. Maybe a simple, pretty ribbon. I like white roses, not long stemmed, but not a lot of them. (Growing up, we had a hybrid tea rosebush that I got to pick, called Honor. It had gorgeous white blooms, and a wonderful scent.) One is better than a dozen, IMO.
A nice bouquet of brightly coloured Gerbera daisies.
Peonies. Big pink, white or red peonies.
Failing that, something tropical, like flame lilies, bird of paradise, Anthurium or Frangipani.
If it was from my husband- Vendella roses (which are a sort of peachy white) because they were our wedding flowers.
Try going to a locally-owned florist and tell them that you want something impressive and unusual, and see what they have. You might find something that she’ll have never seen before, but will love. (Extra points for creativity!)
Most women (who aren’t allergic) like bright colored and lovely smelling flowers, so go with big and bright and scent-y.
Good luck! 
Zinnias are awesome.
Mini sunflowers too.
And white roses.
I especially like when they come in a vase already so I don’t have to say “keep your pants on for the next 20 minutes while I properly cut all of these flowers on the bias under water, remove low leaves and arrange them just perfectly in this vase I had to dig out of the cupboard. Once that’s done, then we can hop in the sack.”
Here’s an idea, what else blooms at the same time pussy willows bud in your area? Apple blossoms, cherry blossoms, crocus, daffodil, tulip, iris? I don’t think Morning Glories start blooming at the same time as pussy willows bud though. Hmmmm… So, what blooms at the same time as Morning Glories? Zinnias do, so do cornflowers. (And they are wildflowerish.) What about portulaca/moss roses? You could get her a pot of those, and they self seed very nicely if you scatter a seed pod or two from time to time. There is a great variety of color with moss roses, at least in my experience. You never know what colors you’ll be getting, and that’s neat. It’s like zinnias.
ETA: Does she think cattails are neat? 
(They make a horrible mess when they finally come apart/go to seed though.)
Peonies.
A nosegay of very densely packed lilies of the valley sprigs, with the leaves as a backdrop.
VCNJ~
Calla lilies or sunflowers are nice…