They could try and get their travel agent to advocate for them with the cruise line. Unfortunately in this situation it's going to be unlikely they can remedy the situation.
These situations are precisely why the cruise lines (and 3rd part companies) offer travel insurance.
They spent $4900, and chose not to spend maybe $400 to insure they were covered for a wide variety of things.
This may sound harsh, but people seem to expect the cruise lines to cover the risk of every possible occurance which might occur for EVERYBODY who books. They want them to take all the risks of things going wrong, probably just because they view the cruise lines "deep pockets".