Default How to unlock phone?

I'm not sure if the question is right. I'm very ticked, since I was ripped off. My girlfriend wanted a G2 since it came out and she found someone who was selling it for 150. She picked it up today and can't use it because the guy owes money on his bill. So since they won't switch it over, I was wondering if there's a way to make them think it's a different phone? Or anything I can do so she can use it.