I'm sad to hear what you experienced and I hope more Christians would follow the example of Jesus (calling Judas a friend) and Paul in treating people friendly and/or at least with respect as long as possible and leaving judgement to God ("for Demas, because he loved this world, he has deserted me and has gone to Thessalonica.", "Alexander the coppersmith did me great harm; the Lord will repay him according to his deeds.").

Sometimes we are forced to stand up for ourselves and to speak out loud and clear. I don't think someone admitting that they don't believe and leaving peacefully is such an instance as long as they don't try to lurk around to pry out or take advantage of others.