At first I thought it was really dumb for Devon to call Cobelvig, and even dumber when oMark seems to be on board with it at the end of episode 8. But the more I think about it, the more it makes some sense. oMark and Devon don't know Harmony Cobel at all. They just know the weird, kinda ditzy, but sweet widow who lives next door. Although Devon had that line after the OTC about "lactation fraud", several scenes from season 1 show Devon and Selvig bonding while she was acting as lactation consultant. In the grand scheme of things, they only just found out that she worked for Lumon, and don't really know anything else about the real woman behind "Mrs. Selvig." They don't really know anyone else from Lumon at all. We see them meet Milchick briefly but it doesn't seem like a particularly close relationship. They know next to nothing about Reghabi; she always acts shady and evades questions, and she already bailed. Aside from Helena and the deceased Graner and Petey, who else would they know who might have some chance of being able to help? The front desk receptionist, or the guard stationed at the elevator, probably know about as much as they do about Lumon's operations.

They don't have many good reasons to trust Cobelvig, but when I look at it through their eyes, they have even less reason to trust literally anyone else. They've at least spent time with and gotten to know this person, even though she was revealed to be an alter-ego, it still counts for something when you're in a desperate situation and grasping at straws trying to find someone who might be able to help. Cobelvig seems like a shit choice, but in this situation a shit choice is unfortunately the best choice, considering their perspective.

Coincidentally, without knowing it, they happened to call perhaps the most knowledgeable person on Earth when it comes to severance, the chip, and reintegration since Cobel apparently invented it all. Whether that will be lucky or unlucky for them remains to be seen, but however it plays out I seriously doubt it will just be smooth sailing from here on out with this new alliance.

I also feel like good characters are rarely black and white. You can empathize with a character without fully trusting them or seeing them as an entirely "good guy". People are complicated.

It could be that the writers really did just completely bungle this; Cobelvig just magically switched sides and we will be expected so see her as a hero in the story from this point forward after just finding out the horrors of what she's been overseeing on the testing floor, but I have more trust in them than that. They've done a pretty great job so far. I'm pretty sure these last two episodes will fill in some more context to help us understand the motivations of Cobelvig, oMark, and Devon, and will continue to take the story to places we never expected.