What kind of cookbook? General, ethnic cuisine, specific ingredient, recipes from his restaurant, desserts? He needs to define his theme and see what's already been written.
Some general suggestions from a cookbook addict:
--New, flavorful recipes. There are 10,000 choc chip cookie recipes, how to roast a chicken recipes, etc, out there and I don't want a cookbook with more of the same.
--Lots of pictures, including any techniques that are difficult to narrate.
--Traditional organization. I hate cookbooks with recipes organized by season, color, or shape of ingredients.
--Substitution suggestions for exotic ingredients that I can't buy within 100 miles of where I live.
--Proofread and retest all the recipes before publication (this would put you way ahead of many cookbooks on the market).
--A user-friendly binding so the book will actually lay open and flat.