There are several cellular companies operating in USA like AT&T, T-Mobile, Verizon etc. It is necessary for any Pakistani Mobile company to have services activated for any particular United States company.
Like Mobilink has their SMS services activated for AT&T so anyone using Mobilink can easily send a message to a USA AT&T number.
It’s better to check with your friend or special one about which company they are using for cellular services and then check with your local company website or customer services whether you can successfully send SMS or not. You can check the list of collaborations on websites in list Mobilink, Warid, Telenor etc. The mobile companies will not guarantee that the SMS will be delivered to the other party in USA but if it is present in the list you SMS most probably will deliver.