What's her natural depth and tone? What colour history does she have- ie is it coloured over lots of times and liable to break if bleached up etc? What's her hair type?
Also, what tone of blonde does she have in mind?
If it was in good enough condition, I would strip the colour out and see what you are left with. Then depending on what she wants and what you have infront of you, you can decide on what way to proceed and what toners/colours you need to do.
I recently went light from 66/44 and 44/44 Koleston Perfect, mine was stripped with Colour remover twice, then 12/1 40vol tint all over for 60mins, then full head meche bleach 20vol and colour touch + 77/07 on rest, it gave me jenifer aniston kinda colour. my hair was virgin hair before the copper colour before though so was in fantastic condition, although not so much now lol.
Best thing you could do is tell her it is a gradual process and take it in stages. And make her invest in some good treatment products and to look after it so it's in good enough condition.
Also, take a few test pieces first to see what way it turns out and the condition etc
Good luck x
