So he's hired a professional photographer and also wants you to come along? Two problems there, most professionals contracts will specify no other photographers, and even if it doesn't you'll ruin the other photographers photos if you're in the same area. Two cameras = people don't know where to look = photos ruined. If there's a professional, do them the courtesy of calling first. If it was me I'd say please don't pick your camera up or stand near me while I'm taking photos as you will ruin a lot of photos, and ideally leave your camera at home. If you must take your camera don't take photos of the same people the professional is, or go anywhere near them.
Contract... photographers toolkit website has a good one. Buy it, it's cheaper than hiring a lawyer to draw one up. Wedding photography is a business that requires investment. If you want to take photos for your friend you don't need a contract.
Final photos: sRgb jpeg 3600x2400 Q10/80%. My culling strategy is I give the customer every photo I took that I'd want if it was my wedding. Each image I give to them has been processed to look its best, in ACR/LR with only 1% of images going into PS.