Is the owner of a television sinful for his colleagues watching forbidden things on it?

You must advise your companions to stop using the device for what Allah has forbidden. If they insist, you must prevent them from it, even by taking it if it is yours. Otherwise, you would be sinning, due to Allah Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression," and due to the Prophet, peace and blessings be upon him, saying: "Whoever calls to guidance will have a reward similar to the rewards of those who follow him... And whoever calls to misguidance will bear a sin similar to the sins of those who follow him."