Is there a sin upon one who did not alert a worshipper who, out of forgetfulness, added an extra prostration during the final tashahhud?

The worshipper who adds an extra prostration to his prayer should be alerted to perform the prostration of forgetfulness (sujud al-sahw), as this falls under cooperation in righteousness and piety. Some jurists have mentioned that it is necessary to alert the worshipper if he adds something that necessitates the prostration of forgetfulness, whether the alerter is a follower (ma'mum) or not. Sheikh Ibn Uthaymeen views alerting as obligatory based on the generality of Allah's saying: "And cooperate in righteousness and piety."