Can I cancel a plane ticket for someone else without legal ramifications if I am the one that purchased it?

I bought a plane ticket for my girlfriend to come visit me from Ohio to Hawaii. Come to find out she has been seeing someone else here in Hawaii. She broke up with me the day after she came and went to her new mans house. I paid for the roundtrip ticket for her to come out. She obviously has used the first half. Can I legally cancel the second half of the trip? I already called the travel agency and they will allow me to use the credit for myself up until May of 2012. I want to cancel the ticket and she can get her own to go back but I just want to make sure I am not over stepping my boundaries.