Luckily, there are thousands of stunning and wonderful places to go to in Greece.
It seems to me that merely getting onto and off of a cruise ship repeatedly would be tiring for your friend. Similarly, public transport, ferries, planes, etc. would be taxing.
To me, Greece is tailor-made for car travel. Whether or not you are on an island or the mainland is entirely inconsequential. Anything you can find on an island is available in abundance on the mainland.
With a car, you can drive right up to the entrance of more mind-blowingly beautiful and historic sites than you could imagine. From there, your friend can do as much or little as is possible and you'll be off to another.
I might suggest getting into a car at the airport, driving towards Corinth with a stop at the canal. A mere stroll. In Corinth itself is the ancient site but that and the museum is flat.
Then, go to Nafplio. Stay in the city center. You can explore the entire town easily on foot and it's flat, too. If you friend could handle walking down stairs and not up them, take a taxi to the top of Palamidi Fortress and walk down to town.
From there, go to Monemvassia. Find an accommodation as close to the gates as possible. Yes, this town is hilly and cobblestoned but the entrance is flat and from the entrance you will still have access to plenty of thrills to occupy yourself for a couple days.
Similarly, on the Peloponnese Peninsula, Ancient Olympia is flat (though large in size, a few square miles) and includes a wonderful museum.
The sheer distances to be covered in Athens are great, no matter what you want to see. Therefore, I would strongly recommend to just get in a car and call all of the shots yourself. It's easy to do this in Greece.
