Goldwell Colourance 10P mixed with 8SB can work quite nicely to add a grey tone. I would add more of the 10P than the 8SB as it can grab quickly depending on the porosity of the hair. I would definitely recommend doing a strand test prior to the toner to eliminate any uncertainty.
It may also be an idea to ask your client to show pictures of the grey she likes? Although she says she doesn't mind what shade of grey, they can vary a lot in terms of how light or dark they are and I imagine she does have a preference even if she doesn't express it beforehand. It is always best to be on the same page as a client to help ensure total satisfaction for both sides.