A big part of a cruise, especially on the major cruise lines, is social interaction. When your daughter and her friend are on the ship, they will interact with people of all ages from all sorts of countries. In the teen scene, there will mostly be Americans from all over the US, but there are also many Europeans and even some South American teens. With a broad, diverse group of teens like this, your daughter and her friend will learn about their cultures and their dialects, and become more humble themselves and all they have! Aside from the teen interaction, the cruise staff come from ALL over the world! On my cruises, I have had waiters from India, Turkey, Brazil, the Phillipines, and France! At dinner and during different events around the ship, your daughter and her friend will be interacting with the adults, whether they are ordering their dinner choice for the night or learning how to do a certain dance during the ship's pool parties. This interaction builds respect between the children and their superiors. By going on a cruise, they will be able to show more respect as each ethnicity and diversity shows their utmost respect in a slightly different way. They will be worldy educated!
If you decide to get off the ship and go on any excursion at port, they will learn an enormous amount of information about the specific island they are on. When we went to the beach at St. Maarten, our taxi driver was a native who told us all about the island and about the history of the events on the island! (We've been there twice and both times the taxi driver was very knowledgeable!!) Because they are both from a small town, this experience will be once in a lifetime!
Let me know if you have any specific questions or if you need some more strings to pull for the friend's father!!