It looks he had all the time still the 'old' cookie in the browser stored, which had no expiry time of 6 hours. Changing the OS, made the cookie lost and replaced by a new, which expires.
I can confirm this theory.
In my case, I didn't even change browsers. I just cleared cookies because I was dealing with an unrelated problem. Then after that, my login sessions started timing out, whereas before, I hadn't done an explicit login from that browser for months if not years.
