If it is a Yid, make sure you got to Beis Din. Taking a Yid to arkoyos (goyish court) is a massive aveira. (I think the halacha would even allow someone to kill someone who did that.)
You are then, hereby, invited to come and kill me. I once called the cops on a Jew (well, his car, specifically) without asking permission from a Rav or a Bais Din.
The Wolf