As with loshon hora, one may not relate rechilus for a constructive purpose if this purpose can be accomplished through other means (see Day 46).
Of course, easiest of all is the direct approach – informing the potential victim of what is being plotted and allowing him to fend for himself. The Torah, however, views this avenue as a last resort. If the option exists, one must protect the victim without his even suspecting that someone is attempting to harm him. (This option does not apply where it is important for the potential victim to be on guard should the other party attempt to harm him at a later point in time.)
Obviously, there is a limit to how much effort one is required to invest in protecting his fellow Jew from harm when the person is fully capable of protecting himself. If, in order to avoid involving the potential victim, one would have to dedicate an unreasonable amount of time and energy, it would be permissible to inform the person so that he can protect himself.