To the people that find it difficult to believe Devon's logic

Take a step back and consider a couple of things that could be going on from Devon's perspective that might prompt her to reach out to Cobel:

 

  • She's just recently found her brother in crisis after a brain surgery. A surgery that was performed in his basement, specifically by someone she does not have prior experience with and who is being extremely cagey. She's not equipped for this, so a bit of impulsivity and irrationality isn't unsurprising.

 

  • She's just recently given birth. That's no small task, and neither is caring for a newborn. It's physically and mentally taxing, and so, again, a bit of impulsivity and irrationality isn't unsurprising.

 

  • She's had experience with Cobel, more than with any other Lumon associate (as far as we know). As far as we know, the majority of these experiences were positive. Despite the betrayal, Cobel still helped relieve some of the stress of motherhood, and it actually feels almost natural for her to gravitate to her out of desperation in this situation.

 

  • She's not fully aware of the inner workings of Lumon (as far as we know). She only really knows that Cobel was Mark's boss, and probably expects her to feel scorned by Lumon for firing her, and, therefore, more willing (and able) to undermine the company.

 

Devon: The floodlight thing was a bust, right?
Because I have had an idea that is a little dumber.
Mark: Okay, Devon...
Devon: Do you remember the rich lady from baby camp?

  • She was coming to Mark with a plan to visit the birthing cabins, because she's also desperate to confirm that Gemma is alive. She even frames it herself as a potentially dumb idea. It's possible that she intended to discuss reaching out to Cobel for help with Mark, and the ensuing desperation makes her act on it impulsively.

 

Devon: What? Why-Why do you have equipment in Mark's basement?
Reghabi: Your brother's reintegrating.
[...]
Reghabi: It's what he wants.
Devon: What he wants is to know whether Gemma's alive or not.
Reghabi: She is.
Devon: Please don't mess with me.
Reghabi: She is alive.

  • She realizes that the reason Mark is reintegrating is because he wants to find Gemma, and is likely even more emboldened to do so herself after Reghabi assures her that she is.

 

Devon: Is he okay?
Reghabi: He's stable.
[...]
Devon: When will he wake up?
Reghabi: When he's ready to.
Devon: Great. [sighs]

  • She's assured that Mark is stable and will wake up. She relents and seemingly chooses to believe that Mark will be okay.

 

Devon: You really think this is gonna work?
Reghabi: There's no other choice.
Devon: There's an innie cabin.
[...]
Devon: It's a cottage where severed people become their innies...
[...]
Devon: I could call her.
[...]
Devon: She might know how to get in the cabin.
And, fuck, she might know how to get to Gemma.
Reghabi: She's Lumon through and through.
Devon: It's a risk. I get that.

  • Now her goal is to keep Mark from continuing with this experimental procedure because she's afraid that Dr. DIY is going to get him killed. She doesn't know a whole lot about how severance works, and assumes that they could just walk into the severed birthing cabin to get confirmation/information from Mark S. about Gemma. She's taking a gamble on Cobel being willing and able to help them, because the alternative is Mark seeking out more brain surgery.

 

TL;DR: Devon reaching out to Cobel isn't all that far-fetched when you consider the various factors influencing her thinking. She's desperate for a way to find Gemma that doesn't involve more brain surgery.