>>149127907 (OP)It's actually a very good moral conflict. Because obviously, yes, she should've, right? Hell, all of them should've. Dale's kind of a piece of shit, but he's absurdly loyal to Nancy and, for all his faults, a devoted father to Joseph. These people are his friends and neighbor's keeping a horrible secret from him. His own ignorance and blindness only add to this, making you feel, in principle, very sorry for him.
But then there's all the particulars of the actual relationship. Dale is just a ridiculous man, it's a joke he hasn't figured it out already. As
>>149128557 said, it's easy to imagine he wouldn't believe it even if everyone calmly explained it to him. And while what Nancy did is very, very shitty, it's not like Dale wasn't otherwise kind of weird idiot who left her way too lonely. That's not an excuse, but it's clear Dale repairs their relationship over the course of the show in a way that he was not doing around the time Joseph was born. There's also the other practical aspects. Nancy has, or will I can't remember the timeline, stopped seeing Redcorn. Joseph sees Dale, and only Dale, as his father. It's fucked up, but things largely worked out, so why rock the boat?
Really, the most important part is how the show portrays the trials of trying to maintain a community. It's all well and good for us to say, and not wrongly, they should've told Dale. But they have to live with it, and him, and it's really, really hard to see how doing the "right thing" would actually result in fixing anything. By that point, anyway. It's unclear, I think, when everyone learned, but the biggest problem is they let it fester for so long that not ripping the bandaid became a much more attractive prospect.
I think I still would've told Dale, but it's hard for me to know what else I would've done and it would've been a hard choice.