By the way, it's pretty much full sun here in Zone 9.

Fleemo,
WHen the rain hits the top, it runs really fast down that hill, that is why he will have a hard time keeping grass on it, not impossible, but I can't even imagine mowing that.
The idea of gardening on a hill, (he has a great hill), is to slow down the rain.
I would plant some small trees, and under the trees, I would put rocks, sort of make it look like a rock garden.
The trees will slow down the rain from the sky and also slow it down when it is running down hill.
THen I would plant some shrubs, med sized and some tall ones, then I would plant some ornamental grasses, (there is a great variety out there), and finally, I would plant some flowering perenialls, like lantana, daylillies, St. Johns wart, ect.
I don't rake the leaves, I let them stay on the hill and rot. So, make sure he picks some decidious trees, and some evergreens, like some kind of cypress.
Try to get him excited about it, and start with the trees first, all those plants are expensive, especially when the hill is that big.
It's all worth it in the end, each year everything will grow in more and look nice. It takes a while.