I think it would be fair to say that Zimmerman had a lot more to gain from this by telling a lie than the girl did.
I don't know about her credibility, as I think such a thing is hard to do without knowing her past, but I find it really hard to believe that Zimmerman didn't grab Trayvon. Having a gun on your hip makes you feel like a bigger man than what you actually are and as was stated in court, Zimmerman was a horrible fighter. He knew he had that gun to back him up and it gave him the courage to do something he normally wouldn't. Or at least something he didn't do on any of his other calls to the police about suspicious figures.