I would have them look at their booking confirmation and travel summary under passport requirements. They will find the statement below. If they think getting turned away at the port is less trouble than expediting passport renewal, that's their choice.
PA S S P O R T R E Q U I R E M E N T S
A passport book is required and must be valid for six months after the completion of travel. Please ensure the name on your
passport matches your name as it appears on your booking. Additionally, make certain that your passport contains blank
pages for entry and exit endorsements and any visas that may be required. If necessary, allow sufficient time to renew your
passport and/or obtain additional pages. A passport card will not be accepted.
There may be occasions where we retain your passport at check-in. This is to ensure that you are not inconvenienced by
immigration clearance during your cruise and where face-to-face inspection is not required.