If she wants to go all over blonde including the roots, I would paint bleach or foil, whatever method you use, and start with the dark parts that are really grown out. Once these darker pieces lifted enough, paint the bleach on roots. That will take her top to the blonde she wants. Of course have to tone.
Gretchen's hair I would be curious is she has balayage or foils. I ask this because several of my ultra blonde balayage clients have these low lights when they style their hair differently. In fact what looks like low lights are actually the underside of the hair. When I apply bleach to the top of some pieces of hair, I don't always saturate it with a ton of bleach. So the underside in some pieces appears sandy. Natural low lights in a way. You can really see them when they braid the hair or wear a style with underside of hair showing.
As for the length, she actually is nearly there. I have several clients like this. I wouldn't add low lights just yet. Your client already has low lights. Those are the darker pieces. With your eye, you can pull out a few pieces and balayage or foil. Her hair probably lifts easily especially the lengths. If you foil some chunky pieces, could probably even use a volume 10 with bleach and not rush it.
And the length will need more toner with ash. This will counteract sort of sandy color. (I'm in same boat with a new client who wants hair like her friend. The length is sandier than her top. The members here helped me a lot with new L'oreal toners I'm just starting to use. The Dia Light milkshakes.)
I always tell clients also that this is not a 1 salon visit. The length may take a few visits to get exactly where she wants it. I also encourage them not to wash everyday and limit heat and dryers to once a week if they can. Monthly root touch ups are high maintenance. If not done correctly, there can be breakage. One reason many ultra blondes wear their hair parted drastically and covering forehead. Cover up breakage.
Just my opinion x! I'm sure others can weigh in with their techniques as well. With these clients I always make sure that they can't rush something like this. But honestly, your client is nearly there! Just make sure she knows you will work up to the blonde she wants.
