island hoping

A guide book? I mean, they are 30! I always found the Croatian Tourism Board website helpful, plus each town/island has its own website.
All they need for island hopping is the ferry/catamaran schedules: Jadrolinija and Krilo are the primary operators. Schedules are reduced in September, but they can still get among the islands.

We enjoyed the islands of Losinj ( less touristed) and Korçula. We went to a vineyard on Korçula as well as spent a lot of time exploring the old town. Very scenic.
Hopefully, they will enjoy island hopping in addition to the interesting mainland cities and towns.
They must have had an idea about the islands or they wouldn’t have planned a trip there.

Would you recommend a central location to stay or move around?

It depends--we don't know where they are departing or arriving. Sometimes a place lends itself to being a base (for example, Split, if they want a larger city but also lots of options for different island day trips), and sometimes it is better to move rather than have a long day trip, or if the ferry schedules are not conducive to a day trip.
Dubrovnik-island-Split is a common first itinerary. Zagreb is a common starting point, and in that case, Zagreb-Plitvice park-Split would be a good plan. Along the coast, it is really all about the views, nature, so they can't really go wrong. Brac is popular for windsurfing.

How much time are they talking? A week? Two, Three? While Hopping implies "quick" the reality is not so quick, really more like 3-4 days per Island stop.

Have they mentioned any place in particular? The popular places tend to be between Split and Dubrovnik (Brac, Hvar, Korcula, etc.) though further North are places more off the beaten path.

Rick Steves’ guide for Croatia is a must, and they must read it for themselves! Croatia is a tough place to travel quickly as “hopping” implies. Ferry schedules can be mystifying.

They will be there for 7 days arriving in Splitz and have no idea what islands to go to. We are all lost!

Tell them to Google Zlatni Rat beach in Brač (near Bol). Absolutely gorgeous place. I took the bus there from the ferry landing in Supetar. Split is a great place from which to catch the ferries to various islands. Maybe they would like Hvar as well. They are old enough to know how to do internet research, right? Another good point is the public library (free travel books). They must have picked Croatia for a reason, so I would let them work it out.

It's been a week -- do they access to a guide book or internet access? I just don't get how asking strangers is easier than looking at a map or in a book that is designed to suggest itineraries. Split is near three major islands, and ferry schedules show which others are accessible.