I am just conjecturing, but I think you got that response because Arizona is one of the top astronomy schools and they might think you are trying to get into astro through another route. I'm bot saying that is true, but it is probably an understandable concern that they may not get you to work with one of the professors in their department.
One reason why that other graduate student may have worked with the astronomy professor because Arizona makes you choose a major and minor. She could be minoring in astronomy and that is the professor that will sit in on her qualifiers for her minor. I think your best bet is trying to get in contact with the grad student and see if this is an unusual partnership. If it is and you are serious about coupling your work with astronomy you may want to consider another program.