Is a young man considered sinful and responsible for a girl's suicide because he cut off communication with her after they engaged in conversations displeasing to Allah?

What the young man did by cutting off communication with the girl, out of fear of Allah, is what is required by Islamic law. If the girl commits suicide, the sin of that is upon her alone, for Allah Almighty says: "And every soul earns not [anything] except against itself, and no bearer of burdens will bear the burden of another," and for the saying of the Prophet, peace and blessings be upon him: "Indeed, no offender commits an offense except against himself." He must offer sincere repentance and observe the Sharia guidelines in dealing with women.