The yellowishness with darker green veins is called interveinal chlorosis and is a soil minerals problem. The browning looks like it may be related to this condition. At any rate he can try taking samples of affected parts to an independent garden center in the vicinity for comment - they may see a lot of this in his area. And will be likely to have recommendations of what to do about it.

Also garden plant problems in the US can be identified using the resources at USDA Cooperative Extension Service branch offices. Try searching "california extension".

I would make sure that you do not water leaves ever as that helps fungal issues develop on leaves. Also, try not to overwater as saturated soil makes it difficult to absorb minerals and its humidity helps fungi develop. Check your soil pH while you are it... to make sure the soil has not become "too acidic". A formal soil test every 5-10 years (or if you have never done one) is a good thing. Camellias have shallow roots so make sure that you do not have other stuff creeping into general area. When these infected leaves fall, throw them into the trash so they do not spread the fungal spores.

Southern California soils are typically near-neutral. Further, this plant is close to a masonry wall.

Doubt those are fungal lesions. too dry in SoCal for that to occur very often.

The shrub also looks leggy and sparsely leaved.

Overall, I'd say short of water, possibly also fertilizer and light.

Jean,

who gardened in SoCal [Long Beach] for 30-some years, including some healthy camellias.