Will a young man be sinful for sending a video clip that later turned out to contain indecency, and will he be sinful for his friend watching this video?

As long as you sent the video while unaware of its包含that which is forbidden, and you thought it permissible, then there is no sin upon you, Allah willing; for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for their mistakes, forgetfulness, and for what they are coerced into."

However, you must advise your friend and forbid him from evil. Be keen to associate with the righteous and avoid the company of the corrupt; for the Prophet, peace and blessings be upon him, said: "Do not associate except with a believer, and let no one eat your food except for a pious person."