First, you are assuming he only does it for one monster per combat, Which I doubt is the case. Second, you assume it's the last monster in the combat so it's a given the PC's can gang up on it. And depending on the rolls doing this for more then one monster in combat can definitely affect the length of combat...
Hah! You're right, I am assuming that, but you're making the opposite assumption - that he's doing it all the time and it's making a major impact on his combats. I really have no idea which one is the case, so I'm just throwing out what I've done since my early days of gaming. And essentially we're arguing about two different things.
4e for me is no different than any other RPG I've played - on very rare occasions I fudge, giving a monster more or less HPs at the last minute for dramatic reasons, at about even ratios. There's nothing special about 4e that makes fudging like this remarkable, and we can look to one of the hundreds of fudging threads for discussion on it.
My argument is that slight and occasional HP fudging won't change a player's or DM's perception of combat length in the slightest because the amount of time we're talking about is minimal and inconsequential in relation to the combat as a whole. Maybe 2-3 minutes, every few fights. It's irrelevant.
NOW - If we're talking about effectively knocking 5%-10% of the HPs off every other monster, we'd be seeing a considerable difference. I'm guessing this is what you're interpreting the original statement as. Yes, that would be a big change, tantamount to a house-rule, that would certainly influence the length of combat, and you'll find no argument from me there.
In the end, my point is that it's kind of disingenuous to say "IME combat as written works perfectly"... but you're not running it as written, and admit so.
And I'd argue that, by focusing on the minutiae you're ignoring the big picture - that it doesn't matter in the slightest to the overall experience of combat. I mean, you
can take a hard-line RAW approach like you're doing, but other than scoring rhetorical points, it's irrelevant.
Speaking of - Is having monsters retreat running combat as written? I mean, I could cut my combats by a third, easily, by having monsters retreat when the situation gets sticky.
-O