Put yourself in his situation.

'You don't even have to write anything yourself, Lewis. Just sit in with the writer for a few hours, talk to him about your upbringing and experiences, and hey presto, the book'll be on the shelves in a few months. Just sign on the dotted line and collect the 6 figure cheque on your way out. Royalties will be in the post every month after release'

Hard to turn down such easy money, and I'm willing to wager that the vast majority of peeps would do exactly the same.