Hilchos Rechilus, chapter 9, section 6

6. The permission for two people to tell someone who tends to act on his own only saves them from the sin of forbidden speech. But they are still considered as helping a sin, since because of their words the listener does something forbidden. He is not allowed to act on his own until a beis din rules that he may do so.

Besides, it is hard for all the details needed to allow it to come together. It is rare for the speakers to know all the details of the laws, so that they can judge correctly what a beis din would rule.

So one must be careful not to tell things to a person who tends to act on his own without permission from a beis din, so as not to get caught in the trap of people of the tongue. "One who guards his mouth and his tongue guards his soul from troubles."