I need to produce a sponsorship letter so a Pakistan friend can come visit. What would be required in this letter?

Technically, visitors to this country do not require sponsors, but it can help a consular office approve a visa if they know the person has a place to stay while they are in the US.  Since sponsorship is not a formal requirement for visitors, there is no standard format for such a letter.  It would be important just to explain the relationship, and that room (and board) are being offered to the visitor during their stay.