They will turn to their new friends in the group that will understand them and support their decision. Even if they remain with the group for a period of time, they will need friends outside of the group when they decide to leave. Let them know that you care for them and will be there, should they need you. If they have joined a destructive cult you have several options. Some groups display a few symptoms of cults to a lesser degree and do relatively little harm.
People join cults for different reasons.
Some join because they like the social benefits of living in a community. Some join because they believe the doctrine and believe that the group has the key to their spiritual growth. Many stay because they believe the leader is the fountain of truth or because it is easier to believe than to face the fact that they were wrong and now have to face the uncertainties of life without the group. Knowing this, you must decide where you think it is the best interest of the person to intervene, or not. You can have a heart to heart talk with your loved one and ask them the questions suggested above.