How did the cops know you were white when you called? Or, did your gf assume that once they got there, and you told them you were the one to call, they would take it seriously as opposed to if she had done the same thing? Is it also possible that gf was afraid that onlookers would consider it to be a "black issue" if the triangle was black kid/black parent/black caller?
In any case, that's really sad, but unfortunately believable.