What tncruiseman said is correct at least for this year. As long as their visa for the US is a multiple entry/exit visa. Because they will basically be leaving the US again and then returning through customs. But, soon even US citizens will need a passport for Mexico. I'm not sure if that will change the "all you need is a sign and sail card" thing. I have not cruised to a non- Caribean/Mexico/Canadian port aside from Belize (which was the same sign and sail card deal)to say how a true foriegn port would handle things. Best to have them contact the Mexican Consulate and ask...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007