Hilchos Rechilus, chapter 9, section 7
The same applies to informing Shimon that Reuven stole from him or harmed him etc. Also,
one may only warn Shimon if one already rebuked Reuven and he refused to listen as we wrote
earlier in chapter 1 paragraph 3, see there. One must be very careful to not be lenient until one
ensures all the conditions mentioned above are fulfilled for otherwise it is very likely one will
transgress the Torah prohibition of rechilus.