This seems like a good video your hunter should watch.
Its from wotlk, but its still relevant.
ONe final point in doing the kind of analysis I did above... remember that some fights have mechanics that cause a player to have to stop DPSing. For example, on Jin'rokh you can get targeted by Focused Lightning and have to kite the ball of evil around the raid and to the back, then run back. Yes, you can kite and dps, but you're going to be less efficient dpsing and some people focus on getting the kiting right and don't really DPS. It's hard to argue with that - I'd rather someone have a few seconds of downtime on DPS that drop a Fissure in the middle of the area.
So, when looking at how often a CD ability like Kill Command should have been used make sure to allow for things like this. 20 KCs is still too low, but if 45 is optimal and 42 or so is very good if the hunter got, say, 3 Focused Lightnings you might want to think of the fight as abotu 15 seconds shorter (rough assumption that kiting and dropping the orb takes 5 secs each time). That drops the theoretical max # of Kill Commands from 46 to 43 and if we're allowing a bit of slop, then his uptime count should be 39 or 40. In this case it doesn't drastically alter the basic point that he needs to use abilities more often, but....
Finally, of course, if someone dies 75% in you can't count the entire fight as their potential active time. Yes, I know this seems obvious. But, better safe than sorry.