The Thayan Menace said:
Like sleeping in separate rooms ... in a town with an active thieves' guild.
-Samir
It wouldn't have made any difference - all the PCs failed their listen checks, and all their Fort saves (which, coup de grace Fort saves are difficult to save against).
The guy already said he didn't understand the rules. If he'd bothered to read up on the rules for pulling this off before hand, he might have realized that there was a very high likelihood that he would have a TPK on his hand.
The open locks thing was never a problem. The rogue took 20. So, all locks were opened.
The listen check thing was a wildcard. So, there's a good chance that someone would hear it, but at 7th level, the most you're looking at on a Listen check is somewhere in the order of +10 up to a +16 if they have an 18 wisdom, and the alertness feat. There's a -10 penalty to listening while asleep. That has to beat the rogue's Move Silently. A 4th level rogue is looking at about a +10 Move Silently. So, if the PCs are effectively at +0 Listen checks, against a rogue's +10 Move Silently. Even if the rogue rolls a 1 on her MS check, the PC (assuming they have at least 10 ranks in Listen), have to roll at least an 11 to hear her. So, the odds are definitely against them here.
Then, the coup de grace. The rogue is delivering a coup de grace for a 10+4d6+2. That's a Fort save of 26 on average. A 7th level fighter with a 14 con has a Fort save of +7. Assuming the rogue rolls average damage, the fighter has a roll a 19 or 20 on the Fort Save to save against it. Wizards or sorcerers basically stand no chance at all.
So, the mistake here is clearly that the GM didn't fully understand what the repurcussions were.