Are you doing this soon or when she next needs her colour done in a few weeks?
you will prob have to try and foil out the meche again and put conditioner on them so none of the colour gets on them as if you have a nice blonde and put an ash on they would turn grey. So I would foil all those bits out, then apply a quazi all over the rest of the hair to tone it, and stand with it just to watch it. I have never worked with L'oreal so cant advise on a colour but choose a shade that is opposite in the colour wheel and maybe in a level 8 or 9 as ash can darken the hair. If it's a red in it use like a green ash, if copper use a blue ash etc. O rif she wants to keep some warmth mabe use just a natural 8/0 over it and it will take the 'edge' off it for you.
Sorry if I have confused you. I reccomend you get her in and do this before her roots come through again as this will only add to the problem!
HTH x