Taking a client from highlights to a full head of bleach application can sometimes be tricky. The best way to go about it is to first apply the bleach to the root area and then process it until it is lifted to a pale yellow undercoat. Then apply the product to the lengths and ends (condition permitting) this is to lift out all the old darker pieces that may be present. You should then process until a very pale yellow is achieved. After the lifting process has finished a toner must be applied to neutralise out all the remaining yellow. You don't have to use a super ash shade, even a soft gold, beige or pearl blonde can look beautiful.
Unfortunately, if she is a natural level 5 then no high-lift blonde shades will create a pale platinum blonde. It will appear very golden almost yellowish. I would see if she wanted to take her blonde down to a level 8 with highlights. This then could be achieved by tinting the hair without unwanted brassiness.
