Strand test first and do a mild lighter. I'd say the base 5 is probably achievable but I very much doubt the hi lights aswell all depends what she has on her hair is it virgin? If so you will have to place hi lights were there aren't any red flashes. What colours do you use as I can only advise through wella.
Bleach bath to base 6 then weave highlights through using 6% (I would only t section) then flood colour touch 5/0 through the rest of the hair. After the highlights are done, and are a little orange then tone with 10/8. plenty of aftercare treatments! Wella enrich are fab! Or the SP range is even better! Xx
Just incase she doesn't like it, then you can correct it or if she wants to go back dark again just a personal preference of mine I like clients to go middle of the road if going dark to light so they can get used to it , some people find the whole head a bit of a shock if they have been dark for a long time xx