Late to the party, but there is something that needs to be addressed, for the good of the group:
People can change. People do change, they change all the time. Your entire life will be filled with experiences like this that help shape and mold you as a person.
Now, is this 19-year-old girl going to change? Absolutely, 100%. Is she changing for you? No, she is changing BECAUSE of you. Now, there is no guarantee she will change for the better. But by not taking her back, you are allowing her to learn from the experience and adjust her behavior accordingly. This is not to say you are doing her any favors, she will likely be distraught. But she has to live with the choices she has made and the consequences that follow, regardless of the intent.
At the end of the day, leaving her is the most noble thing you can do. It helps educate a confused, selfish young woman about the intricacies of a monogamous relationship while she is at an age where that type of behavior is generally tolerated.
I feel it needs to be reiterated that this young woman is 19-years-old.
To paint with broad strokes, this is exactly what 19-year-old girls do.
Educate her now as to why this is unacceptable behavior. And when I say "educate", it's as simple as never speaking to her again. No discussion is necessary, your silence will be explanation enough. And mark my words, she will change. She will either change her ways so she never has to be faced with this situation again, or she will change her methods of deception. Regardless, there is a change happening.
The only constant is change.