Just remember, Google and Facebook reserve the right to use any photo you upload in any way they desire. This includes putting them in banner ads and selling them to other sites to be used for what ever purpose the site desires (even as the still image for a vide clip even if said clip isn't real which is often the case).
She might not actually do porn, but someone just noticed her images were good and did a little "shopping" with them to help them sell something.
I have had a few banner ads up of people my girlfriend knows, she's noticed them and then stated "they are not apart of that", their image, however, was.
Ask her about it. If someone is using her photo incorrectly she can request for removal.
Possibly ask her using my posts perspective. Covers you if its not and you can be supportive. If she is. You still asking assuming she was innocent.
edit: Bit late but he still asked -_-