What is the ruling on shortening and combining prayers for a resident of Tanta who works in Cairo three days a week? And is he permitted to shorten prayers for life?

Shortening the prayer for a traveler is a confirmed Sunnah if his journey permits it. It is also one of the excuses that allow combining two prayers. Shortening the prayer is permissible even if the journey is prolonged, as long as the traveler does not intend to reside in a place for four days or more.