I agree about the greenboard (I've used durrock in all other water locations)
This specific wall though, is widely open because of needing to deal with the floor.....which means I have to deal with the cabinet.....which then means, I need to deal with the plumbing.....
So it's all open as we get other things done and now, we are turning our attention to this location. I don't mind regular drywall (which is what's here) because we're only using it to close up the wall and keep the insulation in place. She's going to add 3/4" T&G to the wall for the finish. I asked what if we used Teak as I made a vanity top out of solid Teak in our 1/2 bath.... she said no, too expensive. She wants to use 1x6 T&G clear pine so that's what it is. It is likely to get wet "every now & then" but as a practice, I don't see this room getting used much.... even less the sink. She bought too pretty of a porcelain vessel to be doing her plants & gardening stuff in it, besides, she's got a hose just outside for that.
As a practical matter, this sink is probably not needed at all. But, I digress back to.... I tell her she can do what she wants and to that end, I don't care. If she wants a sink, we'll make it happen one way or another.
(then in 4 years, she'll decide she wants to instead change it to a stand up shower, or remove it, or make it a car wash or SOMETHING..... just to "update" things HA!)