Is the young man considered sinful for what he did with the man who invited him to indecent acts, especially if the man abandoned prayer because of him?

There is no need to continue engaging with or sitting with a bad man after realizing his evil intent, and your listening to him encouraged him to reveal his wicked purpose. There is no sin in deterring that malicious individual and threatening to expose him, even if he abandons the mosque. This does not necessarily mean he will stop praying or performing prayers in congregation, and often the reason for his disappearance is his shame over his exposed actions.