Is it allowed in the Pakistani culture for a woman to marry an Arab?

It depends on what culture she is from in Pakistan. There is no such thing as "Pakistani culture", as Pakistan is a multi-cultural country. If she is a from a more traditional/tribal and rural family, I would suspect great pressure against marrying "outsiders" (which not only includes arabs, but anyone outside of their "braderi" - tribal fraternity). If she comes from a more modern, urban family then I suspect they would be more accepting of a marriage to a foreigner.