How can you remove yellow tones in blonde hair?

I know tomatoe ketchup helps remove green in blonde hair, I also know blue/purple shampoo is good. but is there a faster way or some little secret tp get rid of the warm yellownes. Its been bleached twice and tones twice but my friend still isnt happy with the yellowness. I think she wants it a bit more white kind of like gwen stafani colour or something.