Prayer: What is my duty if I see someone making mistakes in prayer?

Practical Laws of Islam as per the teachings of Imam Khamenei

Prayer: What is my duty if I see someone making mistakes in prayer?

English:

Question #725:

What is my shar‘ī duty when I see someone performing some acts of prayer wrongly?

Answer #725:

If he is ignorant about the prayer parts and terms which if performed wrong do not harm the validity of the prayer, you are not required to inform him. However, if it concerns the prayer parts and terms which if performed wrong , even out of ignorance, they invalidate the prayer and make it necessary to repeat it, like wuḍū’, ghusl, timing, rukū‘ or sujūd, then one should inform him about the correct ruling.

-Imam Khamenei, Practical Laws of Islam, Importance and Conditions of Prayer