I don't know about excursion flights to Antarctica, but one factor your folks may want to consider when making their own shore arrangements (i.e. not through the cruise line) - if there is any kind of problem with their shore excursion and they are late getting back, the ship will not wait for them or make any kind of accomodation for them. They will be on their own and out of pocket in figuring out how to catch up with the ship. While do-it-yourself excursions are not usually a problem in most ports of call, a shore excursion to Antarctica could pose more than the normally anticipated difficulties simply because of the nature of the place and the weather.