The quality that made Nancy a good dealer in small-time Agrestic is the same quality that's landed her in over her head since moving on to the big-time border games: A single-minded belief that she can handle whatever might come her way, and the (often reckless) confidence that comes with that. Granted, it's just as often dumb luck and feminine wiles that get her out of those jams as it is quick thinking and shrewdness, but she's managed to sidestep disaster repeatedly mostly on her own up until this point. But I think it's safe to say that things are the worst they've ever been in Nancy's world, both at home and in business, and tonight she took a major step toward extracting herself from the shitpile she's spent the past season burrowing into.
After discovering Ignacio and Guillermo are moving more than just pot through the tunnel–namely, guns, heroin, and unwilling women–Nancy has one foot in the saddle of her high horse when Guillermo knocks her down with what we've all been thinking since this tunnel materialized: essentially, "Well, what the fuck did you think was gonna happen?" As Esteban has what amounts to a don't-ask-don't-tell policy when it comes to his underground business dealings, Nancy's finally forced to decide how far she's willing to turn her head. The answer: not that far. Enter Agent Till, in the first genuine cliffhanger ending of the season. Though even this 180 turns out to be more of a, oh, 120 or so, as it seems the only name she's giving up is that of Esteban's underling and her old mentor, one Guillermo Garcia Gomez.
I'm not sure I fully understand Nancy's plan at this point–while she'll presumably receive immunity for cooperating with Till, does she expect Esteban to avoid any scrutiny?–but I think I like what it signals. After a sorta-scary descent into danger-mongering and kingpin-like aspirations, it would seem that with this move she's finally realized her limit and is backing off. Like she said to Silas in response to his suggestion that they team up to expand his grow operation: "Been there. Done that. Fucked it up, twice." Maybe Nancy is sick of getting burned and is ready to back the fuck away from the stove.
Or maybe not. Seeing as it took only the lamest of middle-school-level goading from Silas to get her to reconsider his offer–"Yeah, Lisa said you'd say no"–I doubt Nancy is planning to give up the game entirely. Maybe she thinks that by handing over a big get like Guillermo, she'll be able to finagle a return to the small time, and maybe even get back to being a mother in the process.