Who deserves to be first and second author? PhD student who collected data, research associate who wrote the paper or supervisor?
Put yourself in the shoes of the PhD student. She spent 2 years of her life conducting a study and collecting data. She needs help with statistics and writing (this is very common). The person helping with this claims first authorship (in my field this is already questionable) and on top of this even wants to give the second authorship to their supervisor. I can't see how this is supposed to be fair to the PhD student. The post-doc at most deserves third or fourth author. Where is the advisor of this PhD student? You are doing their job (and in my field the advisor would be last author).