As I read the original quote, the questioner wants to know the identity of the person doing the asking (the subject), not the identity of the person or thing being asked (the object).

Or to look at it another way, a valid answer to the question would be "I am asking," or "she is asking," or "they are asking," all of which are subjects. It would be incorrect to answer "her is asking." He/she/they/who are a group, and Him/her/them/whom are a group.