Welcome to the forums.
She is correct in saying she is the secretary to someone. She could also say she is someone's secretary. Both are fine.
See here for usage: [Davies/BYU] BYU-BNC: British National Corpus
Hello, please help me. I have a student who always says she´s a secretary to Mr. White. Is it right to say that? Isn´t the term "secretary to" used with a post, not a name, in which case she should say she´s Mr. White´s secretary? Thanks a million.
useful link, Anglika, thanks.