Greek Island in summer for a beginner?
The strong winds might be too much for him in the Cyclades.
In Naxos and Paros the winds are 25-35 knots.
I would think of Rodos island, as you can choose lighter winds near the northern parts of the island, around Fanes, Paradisi and Krematsi. Once your friend gets comfy with stronger winds, he can go south to Prasonissi for nuking winds.
From Athens to Rodos, better fly than spending a day in the sea.