If the hair is strong enough to go down the full head bleaching route, then great. The thing to remember is that anything below a 6 tends to be incredibly stubborn to remove from my experience. Especially since there's a few permanent 5/0s on there. You could certainly use a colour stripper.... I'd expect it to reveal an orange undercoat which you can then do something good with. Why don't you strip all of the hair to a level 7 orange, then do a full head of bleach foils back to back, tint with a level 7 in between, tone your bleach foils with say 10/16 or 10/8, and explain to your client that you can get her blonder on the next full head of foils? She will be considerable lighter after this process anyway! And without the unwanted tones xx