I'll just make a comment from my cosy European perspective. He's still working 9 hours a day. But it's okay, he "doesn't feel as burned out". Seriously, is this what is expected, or is it just the default?
In many cases it is expected or at least the cultural norm. However, I think that there is a growing body of research that shows how unhealthy it can be.
I think 8-5 is actually 8 hours in the US: federal laws guarantee you an hour lunch break, just as in the UK. Of course, I imagine in a lot of start-ups that's ignored or perhaps doesn't even hold.
Ah, so that probably means he's not actually working 9 solid hours per day. That's better. I admit to doing the same, albeit at different times of the day (10h-19h).