Stan wasn't going to pretend like he got it - because he didn't, he couldn't possibly and he wouldn't lie to Richie to try and make him feel better about it. All he could do was be a support for him as much as he could. Maybe that was the point of all of this. Giving him the chance to do some of what he should have had the courage to do before. Be there to actually support the friends he loved so much.
He hated that this was a weight that Richie was carrying, and one that he seemed to think he needed to carry on his own at that, hiding in the bathroom with it like that was the best way to handle this. Because yeah, this conversation? It was going to be absolute shit, but sometimes that's just the way things were. And Richie? He'd survived some shit. Stan didn't know all the details but he knew that much.
"You're not an idiot, Rich." He said first, needing Richie to know that, they might give him a lot of flack for being an idiot in other things, but going that night wasn't what made him an idiot. It hadn't been Richie's fault he hadn't remembered what they'd left behind, anymore than it was his fault that all of what he'd lived through weighed on him so heavily now. And Richie was right, he didn't see it - not any of it, not all their friends back together in one place again, not the fear on their faces when they realized what they'd walked into, not Eddie's last moment's, not the look on any one of their faces when they'd heard what he'd done too. He'd missed all of that, but trying to protect them, he'd protected himself too, and he knew it wasn't fair.
"I don't think any of this means you've got to just let go of all of that." Quite frankly Stan would have been incredibly impressed if Richie had managed to just push all the rest of that to the back of his mind to never think of it again. "I think it just means you have us to lean on when all of this does start being too much."