Interfaith prayer can be acceptable when praying for or with someone of another religion, but becomes wrong when the act affirms their belief system as true or acceptable to God.

Mike answers situationally. He would not refuse to pray with or for a person of another faith in contexts where prayer is an act of compassion or witness. However, he draws the line at participation that implies affirmation of the other person's religious beliefs as valid paths to God. The distinction is: praying for a Muslim friend (acceptable) versus participating in a joint prayer that treats the Muslim faith as equally approved by God (not acceptable).