Staining Cedar Deck
He could try a cedar deck cleaner and then a brightener. The cleaner would get rid of grease/dirt, while the brightener would get rid of the graying that was caused by the tannins in the wood. It usually brings the nice natural color of the cedar back.
Check out SuperDeck. They have cleaners and brighteners. There are also many other manufacturers that have these as well.
Most of them spray on with a weed type sprayer, let it sit (keeping it wet) for about 15 minutes, then hose off. Do small sections at a time. After first doing the cleaner, followed by the brightener, let it sit 4-5 days or until the wood is down to no more than 15% moisture, and stain.