Lots of questions…
Did they book the flight with the cruise ship company?
Did they book a transfer from the airport to the ship with the cruise company?
if so, then they should be fine. If their flight is delayed or cancelled and they booked these with the cruise company my understanding is the cruise company will get them on board….they might miss a port though.
Are they retired? If so, why put themselves through the stress of wondering if they are going to make it. Fly in the day before, stay at a hotel that has airport and cruise port shuttles, relax, have a nice dinner and leisurely breakfast before catching the shuttle to the ship. Their trip starts a day early.