his name

i recently made a new friend and when she comes over (atleast once a week) she always calls my son Will, we dont call him Will and never have or will and ive tried hinting to her that his name is william but she constantly calls him that. how do i get her to stop it without being rude and scaring her away. i dont have any other friends so i would like to keep her. EDIT: my son is only 9 months old, we dont want to use nicknames so as to not confuse him, he has only just been recognizing his name and we dont want to make it harder for him