real-brisker — I don’t know anything about the halachos of mesirah, but it seems to me assuming both people are Jews you are not talking about mesirah (had you called up the police it would be a different story. . .) — although the victim may have that shaalah.
The issue seems to be one of rechilus (reporting to someone something bad that someone else said/did to them), and would therefore be subject to the rules of rechilus l’toeles. Enabling someone to receive compensation that is coming to them is in that category, provided the prerequisites for rechilus l’toeles are met, including first hand knowledge (you saw it happen), first attempting to speak directly to the wrongdoer, accurate reporting with no exaggeration, there is no other way for justice to be done, you have no ulterior motives, and the person spoken about will not receive any harsher punishment then what is coming to him under halacha. In this case the two that may pose a problem seem to me to be that you didn’t first attempt to speak to the person directly, and the fact that by reporting him he may end up being forced to pay something that is not in accordance with Halacha, as others posted earlier. I don’t know bottom line — ask a Posek! But it is not simply a matter of saying “mind your own business” because in a case where rechilus is considered l’toeles, often you have an OBLIGATION to say it.