Is one who points to a worshipper complaining loudly during prayer sinful, and what is the best course of action in such a situation?

You bear no sin for what you did. The man's action is not from the Sunnah; rather, it is a disliked movement in prayer. You could have spoken with an influential person in the mosque to advise him. Speaking is easy, and you are not sinful for advising. You should lighten the prayer if you lead people, in compliance with the recommendation of the Prophet, peace and blessings be upon him.