ahem. So are you her hairdresser? Are you asking which Boots tint she should use on her hair, or are you using it on her hair for her. Either way, what a shame because if you are a professional shouldn't you be either buying or putting professional products on her hair?
I have a couple of clients that ask me about home colours and I just politely say " I'm sorry I can't advise you as I only know about professional products and nothing about the products sold in Boots". They, more often than not, end up having me colour their hair with the right products. A good product for that type of client is Wella Colourtouch in my experience. I use it on a couple of clients that are 50% grey with acceptable coverage, especially if you shampoo and dry before applying and use 4% instead of the 1.9% developer. Beautiful results can be achieved with mixing too, which you can't do with the boxed colours I don't think. Go on give it a go she will be so pleased with the resulting colour and it is so simple to apply and it imparts such a lovely shine too.
Sorry not the answer you were looking for I am sure. But add to that the choice and range of colours purchased from the professional wholesale outlets far and away out perform the boxed ones in the Chemists.