A disputed saving throw

Recently in a PbP game I run (house ruled Classic D&D), this situation happened.

Context:
The player in question is no stranger to older editions of D&D. He's been playing longer than I have (says he started in '79), and he's played most editions of the game although he mostly plays 5E now.

The game is set in my megadungeon.

To speed up PbP gaming, and to get a bit of rivalry/competition like I read about in the old days, each player runs their own party through the dungeon.

I make no bones about it being deadly. Only one person who signed up to play the game has managed not to lose a character, and that's because he quit as soon as he had his first encounter.

The Situation:
The player in question has his party (all still level 1, with 2 hirelings) exploring the ruins above the dungeon. There's a tower in part of the wall that opens up on two different courtyards at different elevations (it's a hilltop castle ruin with a sprawling dungeon beneath it). The party was at the middle level but didn't know that.

The party thief examines the door for traps, listens and hears nothing.

The player then says that his two fighters "barge into the room" and that the NPC hireling "is on their heels." This is even though, as I said, they heard no sounds from inside and had no reason to expect a creature inside. But if there was one, I guess they were hoping to surprise it.

What was really inside was a 3' wide landing (with no railing...it's a 400 year old castle ruin!) and a 20' drop.

My Ruling: 
If this were real life, there would be a good chance that they would not be able to halt their movement and plunge over the side. But I'm usually generous about these kinds of things. The clincher was that the player said the NPC was "on their heels."

If three dudes are charging through a doorway and there's only about 1 or 2 steps they can take inside before they fall, it seems logical to me that the third guy in would crash into the first two who had just managed to stop short. So I gave them all saving throws with a +2 bonus. Seemed fair to me.

The Result:
One Fighter made his save. The other failed. The NPC hireling failed. The PC was uninjured (9hp), but when I rolled 2d6, of course I got a 9! 0 is dead in this game. The NPC had 4hp and I rolled a 6. Also dead!

The Controversy:
Now I'm OK with how I ruled this situation. It's comical and sad that the fates did this. And it's not the first time this player has lost a PC. It's the third time. But he was apparently surprised and a bit upset at how the situation had unfolded.

My Take:
But really, he could have phrased his PCs' entrance to the tower in so many different ways that wouldn't have required the PCs to make saves to avoid falling. If he'd just said "We open the door," then I would have described the landing inside. He was careless in his orders IMO. And since this is Play-by-Post gaming, he had all the time he could want to decide how to phrase his post.

He seems to feel that I was setting up a "gotcha" moment, and not treating his characters as if they had any common sense. Well, I do make a lot of assumptions for the players in this game. I assume that thieves will be checking for traps when time allows. I assume that everyone in the party is trying to be as quiet as possible unless the player says otherwise. I try to assume competence on the part of the PCs. But in this case, I think an assumption of competence doesn't come into the picture.

Or maybe it's just that I'm a "Mel Brooks" sort of DM. He called it a Three Stooges moment. Either way, it's slapstick. And I'm fine with that. I guess he isn't.

He's not too terribly upset, though. He's still in the game, and rolled up a new Dwarf Fighter to replace the Human Fighter he lost.