Seeing Someone Make Mistakes In Salah
You are not being sinful. But if you decide to approach them, then you need to be wise when approaching others regarding religious details, even with good intentions. A better approach is to discuss the issue with Imam-e-Masjid and let him correct them accordingly. Please watch : https://youtu.be/HkKmAKKLyiE?si=5jcJP7amC-5hez9X
Please also refer to the video below from 1:28:21 to 1:31:05 https://youtu.be/hzSwD2Nq5gg?t=5301