The US Customs and Border Protection agency generally requires foreign visitors to have passports valid for at least six months beyond their intended period of stay. Nations in this exempt group have an agreement with the US allowing their citizens to enter with a passport valid only until their departure date. Travel agents and airline ticketing staff check this list constantly to determine if a passenger can legally board an inbound US flight.
Heard on the job
Check the CBP site to see if his passport is good, I can't remember if Peru is in the six-month club.
Insider tip
Ticketing agents memorize the major countries in this group because boarding a passenger with insufficient validity results in heavy fines for the airline and immediate deportation for the traveler.