Q. I heard its a sin from a Jew to tell on another Jew. Is this right? What if a Jew commits a serious crime to another Jew?. Regardless, of religion, its moral and ethical to report any crime to the police no matter the nature of it or if its from a Jew to another Jew.

A. The answer is complicated because of the many conflicting factors that could be involved.
Horav Shlomo Miller’s Shlit’a opinion is that when the law requires that the respective authorities be informed, it is an obligation to comply, especially when the welfare and safety of children or abused spouses is at risk, or when other defenseless victims are placed on harm’s way.
Sometimes, he suggested, when no immediate danger is expected, it may be wiser to approach the perpetrator first and warn him that if the wrongs he is committing do not stop at once and the victims offered compensation, he will be reported to the authorities. It may be proper also to first inform the parties affected, when not aware of the wrongs committed against them, such as his uninformed employer or neighbours etc. that someone is stealing or harming them, and let them come to terms with the culprit.
As mentioned before, cases differ, and besides the important principle of not becoming a “moiser” or informer, various others, equally or more crucial rules, may also be at play. Therefore, the Rov recommends when it is not an emergency, to consult first with a competent Halacha authority, before taking any action.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a