Sweet Galactus Almighty, Mary Ellen is the worst.
At first, Bryson and Katz try to give her the benefit of the doubt. Sure, she might be a bit of a chatter mouth. And yeah, she might be a woefully inexperienced hiker. She's even a bit cocky, too. Although they want to ditch her (and they, uh, do), they also feel bad for her because she has such a complete and utter lack of skills. In fact, Bryson becomes so concerned for her safety that he can't even enjoy the piece of pie he's been craving for weeks.
After returning to the trail, however, they learn that their fears were for naught. Another hiker (who met Mary Ellen after they separated) tells them that she's been talking major trash about Bryson and Katz, calling them "a couple of overweight wimps who didn't know the first thing about hiking" (1.65.111).
Alrighty then. Well, consider that lesson learned—from here on out, Bryson and Katz will only be looking out for each other.