I’d start with looking at her skin tone and clothes to see whether she suits warm or cool tones.
I think one of the problems with a major colour change is when clients choose the colour based on pics of a celebrity they admire. They can end up with the wrong tones for their skin and eye colour and say, choose a cool brown hair colour that makes them look unwell. Then without realising why it doesn’t suit them, they panic and want to immediately go back to blonde.
Some hairdressers feel anxious about the fact the client is unhappy and so waste their own time and money re-doing the hair for free. If the brown had been a rich warm chocolate colour, it would have looked fantastic on them and everyone would have been delighted with the new look.
Once you’ve settled on the colour you want to achieve, you’ll need to pre-pig with the missing colours (depending on the colour house you’re using), so adding red/orange to your mix. Use a Demi to go darker as it will be easier to remove if she changes her mind within a few weeks.