as an employer , He. can check on her criminal record , drug test and even if she is a spy from another company/vendor. but cannot get into her personal issues/relationaships.
leave your personal matters at home when you come to work.
Thing is sahir doesnt know arzoo has a personal relation with sarfraz ... All he knows is he saw her have a not so normal concersation with one of his vendors whom he doesn't trust. So he is totally justified in digging it, specially when he had directly asked arzoo and he caught her lie!