I don't know that Carnival would be responsible for your privately arranged transportation to and from the port. If Carnival indeed caused your husband to miss the bus, then he may have a claim for his "bus" fare. But it seems you could have sailed and elected not to go. I know, you wanted to cruise with your husband and friends, but in Carnival's eyes, you could have still took the bus and cruised.
But we would want to hear more of the story. Why did Carnival inform your husband days before the cruise that he wasn't allowed to board? And how was it ultimately resolved? Did Carnival refund both of your cruise fares or give you a Future Cruise Credit?
It sucks to miss a vacation, but hopefully the situation is now resolved, minus bus fare.