Emaggie - have you ever seen pics of Derek Jarmans garden, he was also in Kent (dungeness) and the garden was very open to the sea / beach. I currently don't have the book to hand (packed away at the mo) but I remember sea kale, poppies.
Depending on how special the friend (and your budget!) Tamarisk trees are great, eryngium (great seed heads), agave, and I think sage also works in those conditions
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