بسم الله الرحمن الرحيم
It is not clear what type of mistake was committed by the muqtadi. However, if he has left any wajib or similar acts then his salah will be valid and there will be no need to do sajdah sahv (remedial prostration):
ففی البدائع: فاما المقتدی اذا سھا فی صلاتہ فلا سھو علیہ (1/420، دارالکتاب)
But if a muqtadi leaves any fardh (obligatory act) then his salah will not be valid. Therefore, it is better to clearly mention what mistake he committed and ask the question again.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.