This sounds alot like a situation I had with a long time client after we switched from Logics to Socolor. I just couldn't get it right!!! First it was too light, and when I went a shade darker, she claimed she saw brown, and FREAKED! I even contacted Matrix asking for a compatible Socolor formula. Finally, I was able to get it right with just an alteration of the ratio of red/orange, and a couple of drops of color kicks. It was the closest we could get to the Logic's formula, and finally she was pleased.
I agree with Beauty Expert, she should show you a picture. That was what helped me a little with my client. She showed me a picture of herself when she was 19 (she's 64 now). I would say try one more attempt, and then if she just can't be pleased, then I would agree to give up on being driven insane!