What I've encountered is that people with no experience with this kind of work have no idea how much time it really does take. In general, the work other people do tends to look easy. Doing an underpaid half hour job for a friend is one thing, but committing yourself to 6 months work is quite another. Best to realistically estimate how much time you think it will take, you both need to know that.
Maybe do a sample of a few photos and letters, add up the hours it takes, then take it from there. Even if you don't end up doing the work, that sample will serve as a starting point for any quotes he may get elsewhere, which also helps him out.