If the pins are still outside the skin (but under the cast), they will need to be removed in 2 - 3 weeks, which means he will be away when he needs to have them removed. The ship's doctor (if such a thing exists) couldn't do this, and it would risk bone infection to leave them in until he returns.
If the pins are under the skin, then the risk of infection is lower, but if he should get a wound infection (not as bad as a bone infection) he would still need antibiotics.
Your father needs to ask the Consultant in charge of the case if he can go- a junior doctor may not have a clue. The consultant wil also be able to complete an Insurance claim form if the holiday does need to be cancelled.
if you get ill while aboard its all covered under your holiday insurance but i cant imagine the insurance would cover having it removed when you knew it was going to happen (but then should cover refunding the cost of the cruise?)
of course as well as the Dr you pay for all the medication and equipment as well.... even in british water (and british ports) the ships Drs are NOT covered with NHS (and for those with denplan that covers you all over the workd it does NOT cover cruise ships either, as i sepeciafaccly asked last time I was abaord.... my teeth are covered anywhere in the world except aboard a cruise ship!)