I'll take this one from the standpoint of an obstetrician. I strongly support Disney's 24 week rule. The basic issue is that a problem can occur at any time. At 24 weeks, a baby born in a tertiary care hospital equipped with a neonatal intensive care unit has a chance for a healthy, take-home survival. Born in a small rural hospital without the needed personnel or equipment or in the middle of the ocean, the baby has little or no chance for a healthy life.
On one of our trips, I was invited to tour the medical facilities. They are really quite amazing, with lots of equipment that has never been needed (fortunately.) However, they are not prepared to do high risk obstetrics or handle a premature baby.
You friend will be much safer waiting to cruise another time.