I think paint would be plenty, and then the furniture and so forth will come along and fill the place with life, so that the walls won't matter anymore.

In my own patio, which is stucco'd, I have a wall plaque on one wall, but otherwise the light color (a bit more golden than my example above) just gives a sunny backdrop for planters and other stuff. Tell him not to rush -- he may find he needs less ornament than he thinks, and it's not a huge deal to add something later.

Paint, but for heaven's sake, not white, which I think would overpower the room.

In my climate zone (9), creeping fig is a perennial that will quickly take over a wall and is very attractive. It can grow from a planter, doesn't need a trellis for support, and requires average amounts of water and light. I used to have a patio with an ugly wall. I did a quick-fix by painting the wall a deep green and planting the creeping fig. During the first year, while the plant was just getting started, the green background concealed the sparseness of the growth. By the second year, the plant covered most of the wall. It looked lovely.