Salvation is by faith alone. Let each man be rewarding according to his material he uses to build upon the foundation of the Christ. Though he might suffer loss, he will, nonetheless, be saved. Let us all, therefore, build our treasures in Heaven and let God, through the Fire, judge every man's house. As long as Christ and Christ alone is the foundation of what one teaches, seeing that even the Spirit of God points only to Christ, leave that person to his own testimony of salvation and let him teach to the same flock he was once a part of. If one was delivered from trying to obtain their salvation by works, then let that person proclaim to their past flock that works do not obtain salvation. If one was delivered from the influences and/or possession of demons, let that one preach to that sort. If one preaches to those of a dead faith (salvation through head knowledge), let that one teach that to those he once flew with.
Look at the message they are teaching: is Christ their focal point? YES? Then shut up and leave them alone and let them teach that Christ is the way to escape the life they were once of. It is simple a matter of one returning to their past flock and showing them how they escaped through Christ, the Deliverer. Amen.