Question: "What is the best way to evangelize someone who is in a cult or false religion?"
Answer: The most important thing we can do for those involved in cults or false religions is pray for them. We need to pray that God would change their hearts and open their eyes (2 Corinthians 4:4). We need to pray that God would convince them of their need for salvation through Jesus Christ (John 3:16). Without the power of God and conviction of the Holy Spirit, we can never succeed in convincing anyone of the truth (John 16:7-11).
We also need to be living a godly Christian life in front of them, so they can see the change God has made in our own lives (1 Peter 3:1-2). We need to pray for wisdom in how we can minister to them in a powerful way (James 1:5). After all of this, we must be willing and bold in our actual sharing of the Gospel. We must proclaim the message of salvation through Jesus Christ (Romans 10:9-10). We always need to be prepared to defend our faith (1 Peter 3:15), but we must do so with gentleness and respect. I had an encounter with a few cult members once, and a friend who was with me proclaimed the truth, but he did not do so with gentleness or respect. In fact, the cultists were far more "Christ-like" in their attitude and demeanor than my friend was. We may have won the battle for the truth, but we lost the war for the souls of those whom we were seeking to reach.
Ultimately, we must leave the salvation of those to whom we witness up to God. It is God's power and grace that saves people, not our efforts. While it is good and wise to be prepared to give a vigorous defense and have knowledge of the false beliefs we are confronted with – neither of these things will result in the conversion of those trapped in the lies of the cults and false religions. The best and most we can do is pray for them, witness to them, and live the Christian life in front of them – trusting that the Holy Spirit will do the work of drawing, convincing, and converting.