friend from pakistan to visit usa

Visiting visas are given in the discretion of American consular officers. The burden of proof is upon the visa applicant to show that he or she will return to the home country after the period of stay. Factors that may decide an application may be whether the applicant has a good job in the home country, resources, family members, along with the reason for the visit. An additional level of caution is likely given to applicants from Pakistan in light of the perceived problem of the country with elements of its population being security threats.

Your friend merely needs to apply for a B1/B2 visa at the nearest US consulate in Pakistan.  Here is a link with more information: http://islamabad.usembassy.gov/non-immigrant_visas.html