Point out to her that race is a purely social construct, go grab some first year students from the anthropology department if you need help explaining it (honestly, first year anthropology students should be well-informed enough). She may as well call you racist for your preference of hair colour, or eye colour, or the shapes and sizes of facial features for all it is worth.
Alternatively, you could do your paper on how it is not wrong to discriminate on superficial characteristics when those superficial characteristics are the ones that count. It's a terrible tragedy to refuse someone a regular job based upon their skin colour (or eye colour, sex, gender, sexuality, etc), but if you are looking for an employee/sexual partner/actor[footnote]To some extent this can be worked around with make-up and contact lenses and stuff, but if they have a naked scene in a live theatre performance, there isn't all that much you can do to make a woman look like a man (or vice versa)[/footnote]/whatever that has superficial requirements, than discriminating someone because they don't meet those requirements is entirely understandable. I mean, would you think it is wrong to not hire someone for an office job because they can't work a computer? It is a requirement of the role.
)
