TSA PreCheck is applicable to domestic and international flights too, but that is carrier dependent. Flying out of the US on US airlines in the big 3 airline alliances, then it works fine if the passenger has a KTN number (e.g., PASSID) and that number and/or the airline feed about the passengers results in a PreCheck LLL indicator for the passenger's boarding pass.

There are other lawful ways for some to get PreCheck LLL screening even if having a boarding pass for a foreign carrier's flight out of the US; however, most FTers are unlikely to be able and/or willing to pull that off.