Why would and prosecutor even bring Zimmerman up on charges? After they succumbed to the pressure to charge Zimmerman, they have dedicated themselves to persecuting him, no matter how flimsy the evidence.
And how does it help to railroad someone by putting on a witness which can only help destroy your case. There is nothing a witness can testify to that would even out such a devestating cross-examination. The prosecutor had to know this witneess would hurt her case much more than she could ever conceivabley help it. It doesn't make sense.