He did not do the middle tashahhud on purpose
Question

I prayed and I deliberately did not do the middle tashahhud, and I did not do sujood al-sahw (prostration of forgetfulness). I asked Allah for forgiveness after I had finished praying. Is my prayer valid?
Praise be to Allah.Your prayer is not valid, because you deliberately omitted one of the obligatory parts of the prayer, the first tashahhud – this is according to the more correct of the two scholarly views.
And Allah is the source of strength. May Allah bless our Prophet Muhammad and his family and companions, and grant them peace.