How to go from blonde to silver?

[Mrs.Robinson]

I have a client who has dyed her hair blonde, then came to me to dye it silver. It came out grey with little purple and blonde still in it. HELP!!!

 

[ronray]

I have a client who has dyed her hair blonde, then came to me to dye it silver. It came out grey with little purple and blonde still in it. HELP!!!

What was her starting shade and what did you use?