You have a point with the mineral and chemical in the water theory. Malibu 2000 is great for clearing that out. You knew that, but she is preventing you from helping her. Also, tell her to look into getting her water tested, it may be the root of the problem.
Sometimes even if I'm sure that a high lift color should work, the only thing that could cut through the brassiness is bleach. I'm a natural level 6, but could never achieve any kind of a blond without orange tones (I have very strong red undertones), unless I bleach it. Others have tried to convince me otherwise, but I know my hair.