Neither answer actually.
I would say nothing. I would say to her "give me a few hours to do some thinking." And then I would go and ask myself, "what is going on in her life right, now? Is she confused, scared, upset, are there people in her life that are hurting her emotionally, is our relationship damaged and is she trying to tell me she needs my attention and love and might this be a ploy to get me to care about her more." Once I had some ideas I would go and talk to her about her day to day life and ask her to explain to me in full detail what her motivations and reasons are for making a decision like this.
That second reaction is shortsighted, impulsive and stupid. It is more about YOU than it is about HER. It's about how you feel about how what she is doing to you. It is exactly the kind of reaction to a young persons confusions and frustrations that lead to drug addiction and acting out. A parents fist impulse has to to be guidance and guidance is based on understanding the difficulties your child faces, you do that with listening and sending the message that you care far more about how she feels as she moves through the world than how anything she does will make you feel.
Note: I am not saying there is anything she could say that would make me feel good about the decision, but I certainly wouldn't start screaming about disowning her.
While I appreciate the therapy session you more less just proved my point. You implied that if she's (your make believe daughter) is considering a career in porn she must be confused, scared or lost. That's what general society teaches. If a girl chooses a path in porn she must need psychological help.