بسم الله الرحمن الرحيم
(Fatwa: 1365/1365/M=11/1434)
He should do taubah and istighfar extensively and should take care in future that such mistake is not repeated. It is better if he offers two rakahs of salah taubah and seeks forgiveness from Allah.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
