Would he be sinful, whoever invited his friend to watch a song on TV, and then after the song ended, the friend proceeded to watch pornographic channels, knowing that the inviter only invited him to watch the song?

What appears is that whoever guides a person to a song does not sin by that person's subsequent viewing of the forbidden, because he only guided him to listening to the song, and did not guide him to viewing the forbidden, nor did he directly assist him in it. However, he does sin by guiding him to watching the song, due to the impermissibility of singing, and due to what visual songs often contain of nudity and forbidden scenes.