In the real world, all passenger airliners are equipped with traffic alert and collision avoidance systems (TCAS). These are often simulated on VATSIM. If a pilot advises that he is maneuvering in response to a TCAS resolution advisory, do not issue instructions contrary to the TCAS commanded maneuver. However, you should advise the pilot of safety alerts regarding terrain or traffic.
{Reference: FAAO 7110.65 2-1-27}