Is the prayer valid if the worshiper joins the congregation for Fajr prayer after confirming the entry of the time for true dawn (Fajr al-Sadiq), knowing that the Imam started the prayer before the time began?

The time for Fajr prayer begins from the emergence of the second dawn (true dawn) and extends until sunrise. Some calendars may be inaccurate in determining this time. If you confirm that the Imam is praying before the correct time, advise him to delay the prayer until its prescribed time; otherwise, seek another mosque.