If I was in the exact same situation then I would cancel. It really feels like this girl is talking highly of her other date in the hope that you'll back off. Chances are she's just too intimidated to cancel it herself, especially because you work in the same place.
Girls will often speak of other guys in order to gauge interest and create a bit of competition. Though, when done in a certain way they make it so that they want you to win (ex. they'll just make a guy up). This is different because she's already acted on the guy and went on a date with him. I don't know what the extent of your relationship with her is but it seems like she came to you just as a friend. Also, if you were her top priority then she would have found a way to date you before him.
Going by your last few posts it seems like she mentioned the guy, you asked her out, and then she went on a date with him first. Basically, she planned her first date with him after you asked her out. Is this correct?
Assuming she's not interested, she will probably still go out with you tonight. But only because she wants to avoid awkwardness. At the date she will most likely be passive, avoid too much touching, and will probably talk nonstop about how awesome the other guy is. In this case, its not your duty to "show her you're better". In fact, in my opinion you should never compete in that way. You shouldn't have to show you're better, she should already know you are. Know what I mean?
If you realize that all she's doing is talking about the other guy, call the date off early and make up a lie about needing to be home. And then focus your energy elsewhere. In case I'm completely off-base about my assumptions, go out and have a good time. But all the evidence I've seen so far suggests that she's not interested.