How can I find out someone's parents names?

You must also ask yourself are the cops so you will feel better or for her. She needs counseling, she needs to be safe and she needs friends and most of all she needs to feel in control of her life. She may not need the cops right now. You bringing in the cops may make you feel better and might be a noble gesture but it also takes control away from her. I doubt everyone knows about her past life, and she probably prefers it that way.

So do not spring anything on her, if you value your friendship. Friends talk stuff over with each other, they do not spring things on each other, not this kind of thing.

Go ahead and call the cops, but do not be surprised if you loose her friendship. She may even decide to leave where she is now.

Trying to force a victim to go through this is a really bad idea, especially if it's been going on for that long. The only person who can put this girl through this sort of ordeal is herself; don't try to make the right decision for her. I wouldn't go much farther than asking every now and then if she feels comfortable talking about it, but if she isn't, don't make her open up about it. She'll know what's the right thing to do when she's ready. Just be a good friend and be there for her if she ever wants to talk about it.

U dont. U call the cops. They have the authority to do this. U dont.

use her last name or ask one of her older friends or siblings/other relatives?

or wait till she tells you...