7 C's of why people may fall away: Creed, Cost, Call, Community/Christian letdown, Carelessness, Compromise, Conduct.
If you are not walking with the Lord, you are in a state of death. The longer we stay away the more sin takes hold of our lives and the harder it is to return.
If we see this happening to someone we must be like Jesus who would search for the lost and rescue them. It is not just a passive waiting. Be prepared for them to fight back but use wisdom and discretion to reach out in a loving matter. Pray for them that even if you cannot be the one that God will make a way for them to return.