Wait for her to ask it in a straightforward manner. Interrupting a conversation where 'whore' comes up will result in an awkward silence and pre-teen embarrassment.
Liken it to drugs, in an even-handed and honest manner: It's illegal, it can have nasty consequences, but sometimes perfectly average people engage in it and they don't get messed up at all.
If she's mature enough, mentioning the underage sex trade is worth it. Sometimes kids get snatched up (very rare) and sometimes boyfriends pressure girlfriends to have sex with other men for money. In both cases, tell her to talk to you and you won't judge her. You love her very much (and I'm sure she knows that).
And on a semi-related note, Vulvodynia might be worth learning about too. Sex is NOT fun and I spent decades in terror and being told I was psychologically messed up (the then-diagnosis was repressed memories of sexual abuse as a child) because EVERYTHING hurt.
http://en.wikipedia.org/wiki/Vulvodynia
Random addendum, I know. Humor me. ;)