How Should I Respond to Those Who Claim God Told Them to Do Something?

Questions about the best way to tactically and tactfully respond to those who claim God told them to do something and how to get one’s emotions under control when talking to a friend who joined a heretical group that preaches another gospel about her false doctrines.

• What’s the best way to tactically and tactfully respond to someone’s claim that God spoke to them or told them to do something?
• My friend has joined a heretical group that preaches another gospel, and I have sufficient knowledge to talk to her about her false doctrines, but how do I get my emotions under control?