Is there *any* doubt that the girlfriend will lie her a$$ off in order to make the kid look like a fawn in a golden meadow and Zimmerman look like a hunter packing an M-60?
Which is exactly why you want to cross examine her long before trial. This way she can be "locked in" on her story over every conceivable detail. If she lying then a little bit of subsequent investigation will enable her to be destroyed on the witness stand come time for trial.