Bosses sometimes have weak spots. You guys managed to find something the boss wasn't good at. A passive perception of 10 is pretty low, it should be easy to sneak past him if he has no reason to be suspicious.

It's hard to feel like he did the wrong thing if everything happened the way it was supposed to happen. Had he decided the boss randomly had reason to be suspicious without any actual trigger for it, you'd probably feel alot worse.

It might feel weird that you got by so easily, but if it is the legit stats of the boss, you did get lucky that it happened to be one of the least perceptive bosses, that pass without trace is enough passive boost for the check to be literally unfailable.

There is a reason critical failures on skill checks is a common homebrew rule. Sometimes it makes more sense that stuff can't be impossible to fail. But I still know alot of people that prefer to go without it. And in the case of a group check, the crit fail still wouldn't have counted anyway, as group checks are kind of a "best of" type deal. If half or more than half of the group passes, it's a pass. Even with crit fails being recognised as such.