Answered by: Aalimah Sabrina Saidova
Some people who I know pronounce some letters wrong in their Salah from the way they were taught. For example, they say ز instead of ظ.
Is it obligatory for me to correct them or is it enough for me to make dua for their pronunciations to be better?
A lot of the people are a lot older than me so I feel bad to say it to them. Will I be sinning if I don’t tell them?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful.
You will be rewarded if you tried encouraging them into bettering their recitation in salah as making such errors whilst reciting in salah that alter the meaning of the Qur’an in such a way that leads to an incorrect belief invalidates the salah. 
However, it’s not obligatory upon you and you won’t be sinful if you didn’t correct them, but this isn’t the best thing to do.
 Raddul Muhtaar 1/631, Darul Fikr
And Allah Ta’ala knows best
Answered by Aalimah Sabrina Saidova
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham