If there is enough space for an inside mount or if not , an outside one and without damaging the molding . I would consider these , you can keep them up and only see a bit of the folds or none at all,

Cellular Shade Cordless Room Darken 30" Length, Pure White, 22x30 · More Info

Might I suggest a sleeping mask? The windows are too pretty to cover up.

Here is a possible solution:

The Blackout EZTM Window Cover is easy to use and care for.
Simply place the blackout EZTM over the window and press around the edges to seal. The Blackout EZTM folds neatly away when not in use. The special fabric wipes clean with a damp cloth unlike other black out window treatments that require washing and dry cleaning.

Here you go.

http://www.blackoutez.com/?gclid=CI_r94KOn8kCFYE8gQodSoEHsA#.Vk8ga9AzJac

I'm going to suggest Roman blinds as Genevieve has and I also like the idea of installing them higher so that they don't cover the beautiful windows and a will block out light and give privacy when needed.

We agree with Mark, and we love those photos. If the owner does want roman shades, make sure to add blackout liners. Then they really could have the best of both worlds.