What should an ordinary person, not deeply knowledgeable in religious sciences, do regarding a young man who initially showed commitment, then announced his atheism on Facebook after seven years of research, and claims to have convinced dozens of people of the invalidity of religions, given the fear of misleading people?

The intellect is a great divine blessing that leads to guidance and and truth. This friend of yours has fallen prey to the devils among men and jinn due to his ignorance of his religion. He misleads others and boasts about it, and on the Day of Resurrection, he will bear his own sin and the sins of those he led astray. If your religious knowledge is limited, beware of him to protect your faith, and seek a well-grounded scholar to debate him and bring him back to what is right. If he repents, then praise be to Allah. If not, do not grieve over him, for if Allah intends a person to be afflicted with trials, you will not be able to intercede for him with Allah in any way. For such people, there is disgrace in this world and a tremendous torment in the Hereafter.