i had a client with long term build up of colour and the colour i used to lowlight ( 88/0 and 6.3 mixed with 6/7 alt bleach and hi lift) came out much darker and much more ash than expected , even though her hair was strong and very gold.
So id er on the side of caution and explain that you wont get an exact match for her natural because the ash in naturally light hair is vertually impossible to match up , and you will have to have some warmth in the colour to cover the ends . /7 colours by wella are good for covering blonde without pre pigging ,and have a warmth that isnt copper, but id take test strands because you never know how much warmth or depth is going to be absorbed , depending on length and condition of the hair .
You may need to treat the roots midlengths and ends differantly .
Any colour is going to have to be topped up with semis because of fade and to blend in the roots( unless you are both very very lucky)