Showing up sometime between 8 and 11 is unreasonable, because it prevents people to come sooner then at 8. It should have been "sometime between 6 and 11" to be proper.
Also since people know the guy in your story is not working enough, it should be easy to check on him couple of times and then fire him.
Our practice is that you can pick your own consistent start time between 7-9. You can also do things like compress your week to 4 10 hour days or add an hour to get a day every two weeks. We don't want you in the office after 6 on anything approaching a regular basis. You can vary the time by day, which is common for childcare or other outside demands on your time.
If your habit is a 5 hour start window for an 8 hour workday, you're not going to work out.
Also since people know the guy in your story is not working enough, it should be easy to check on him couple of times and then fire him.