Rabbit,
I am so sorry for what you are going through. I have been there a couple of times with my difficult child and online Romeos. Not good. In one situation, she travelled out of state to meet a guy without our knowledge (she was in college in yet another state at the time). That time, we were able to persuade her to return home. She continued the online relationship with him for about six weeks before it fizzled. The next guy lived closer and she moved into a hotel with him. I refused to give her a cent, use my car (she didn't have one at the time) and I would not drive her anywhere. I would see her, have her over for a meal, meet her for coffee,but I refused to meet him. She quickly realized he was no prince charming and moved in with her dad (who enabled her throughout the situation in a big way). She was 19 at the time.
The only advice I can give you is to refuse to enable her in any way. You cannot stop her, but you can let her know she is on her own with her choices. Let her know you love her and that you will always be there for HER, but that you will not help her to live with/go to / whatever / this person at this time. My rule was (and still is) that if a guy is around for a year, things will be different. This kept a couple of of out of state (there were and are still many online romances) guys at bay. "He can visit at Thanksgiving NEXt year". It's worked for me.
Also, I don't know what your home situation is with technology, but I have the only administrative account on the computer, and i've not allowed a webcam hookup. She can skype, but they can't see each other. She doesn't like it but, oh well. It might not work as easily for you - depending on your situation.
You cannot control her choices at this age, but you can set boundaries that make her own those choices.
Believe me, I know how frightening this is. I have lived it and am still living it to a certain extent. Hugs to you.
dash