Is it permissible to mention the name of a TV series to a friend and describe it as beautiful and funny, while warning him about the bad episodes, knowing that the series is free of prohibitions and does not contradict acts of obedience, and would I be sinful if my friend watches it?

If you know that your friend will not watch the forbidden episodes of the series, then there is no harm in telling him its name, while warning him against it. However, if you strongly suspect that he will watch it, then it is not permissible for you to tell him about the series, because doing so would be assisting in sin, and Allah Almighty says: "And do not cooperate in sin and aggression."