Personally, to achieve a 7-1 on a natural base 5 is not possible because of natural underlying warmth. Maybe why her hair has turned out coppery with the home dye.
Ash tones do help to control warm undertones but will not be true unless hair is white or at least a base 7 or above.
I would explain this to her and just use 7-1 on the roots and say the end result will be more like just a base 7 You could try adding a small amount of some ash or blue ash concentrate to the 7-1 mix to help control the warmth, but the result may end up darker than level 7
As for the mid lengths and ends which are coppery. I would apply the 7-1 for the last 15 - 20 mins of development time. But this depends how faded and how coppery the ends are. You may need more than an ash -1 if the ends are very coppery. You may need a 7 with a more blue ash or mix tone to neutralise the copper.
What colour line are you using for this?