I like a girl that has a boyfriend...?

There's a girl at work that I know. At first she was just a friend to me, but I've started falling for her. The only problem is... she has a boyfriend, but he's enlisted and leaving soon. I don't want to come between her and her boyfriend, and by that, I don't want to cause something bad to happen, but I hear alot from her that he's a jerk, but she still seems to like him. We closed the store tonight, and we laughed and flirted and she. I splashed some water on her and she playfully called me a jerk, and I told her I'm the nicest guy you'll meet. And she said prove it... we talked for a moment. it was one of those moments. She left and then I did. When I left, her bf pulled up and they were standing there with their arms around it each other, and it kind of made me sad. So what should I do?