All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
A Muslim should say quite frequently: "O Turner of the hearts, keep me steadfast in your religion." Verily the hearts are between two Fingers of Allah and He turns them as He likes.
No doubt that the deviation of the one who knows the truth and deliberately leaves it is greater than that of the one who did not know the truth and did not get the taste of belief.
Therefore, it is your responsibility and the responsibility of all brothers who know the brother you mentioned to spare no effort to pull him out of the depths to which he has fallen. Try your best to encourage him to end what he is doing now and repent to Allah. Once he was kind to people and had helped to guide them to the right path; now he himself is in need, so they should not forget his goodness towards them; rather, they should treat him kindly and wisely.
A pious person among these people who has a good Islamic knowledge should try to befriend him, remind him of the benefits of being steadfast and the dangers of deviation, etc., and to remove his doubts and misconceptions. The pious brothers should not leave him alone to be trapped by bad friends; rather, they should be with him, spend much time with him, and make frequent visits to him. It is quite likely that he will pass through this trial and will return to the truth. We ask Allah to keep our hearts steadfast on His Religion and return this brother to the right path.
Allah knows best.