Need orientation

Can some one tell me if i have to buy a round trip ticket or one way for my girl friend that will come to marie me and stay here in cancun from pakistan

I believe that she first needs a visa for travel to Mexico. She needs to apply at the Mexican Embassy or Consultant. Depending on the airline, she might need a round trip ticket. But she will be residing permanently in Mexico, right? Tell her to go to the Mexican Embassy to find out what she needs to provide for a visa.