I read that you are helping paint a deck this weekend. You are a saint!
If your friend hasn't bought materials yet, and if the deck is wooden, porch enamel will be his best bet. I bought exterior paint, and it was thick and gloppy (I don't think that's a real word) and took forever. The porch enamel is a bit thinner, but it spreads quickly and dries quickly and actually cures into the wood a bit like stain. I highly recommend it!