Then work on having better time management. Don't create goals that you know your most likely not going to be able to keep. Since he is own boss, instead of doing DAYS on end of non stop work....do a 9-5 , something along those lines. That way, he knows what hours he is supposed to be working on the project and once it hits 5, BOOM, be done for the day. That way, he can recharge. Fuck, even take weekends and hoildays off. He's the only one working on it, he can take those days off to be with freinds and family to have that work life balance. Also, not worry about working himself to the point where his health needs to be a concern.
I don't think anyone in thier right mind would want anyone to risk their health over a fantasy novel where you run around fucking all the hot girls. It's not that important.
I think everyone (And correct me if I am wrong here) would just like him to maybe annoucen release dates, WHEN HE'S DONE. Like once he's completed the chapter, completed the renders, the proof reading, etc....then after uploading then say "It's done, it's up, please enjoy". I don't think that's too much to ask.
That way, he would get a littie bit done each day, he'd pace himself, no more days on end of no sleep beacuse of contstant catch up work, and no more false promises of any sort. Just provide the posting update when you are in the process of uploading it.
If I had a 9-5 job, or a job where I could set my own hours and work at my own pace during those hours...holy fuck, thats the dream right there baby!