Given your description, Theresa, I definitely agree with you it's a local. If someone didn't know the area and was driving around looking to leave Diedre, they couldn't know if a dirt road had a house at the end of it or not. A local would know that this was a little used road with no houses, etc. Also - someone that did not want or plan for her to be found ever.
GHB is also a good possibility based on your description of Diedre. It can be slipped into a drink, and is not very detectable. Especially in Rapids, and around friends in a familiar bar, you wouldn't be inclined to watch your drink. A local she didn't know well could have seen she was separate from Gabe and other friends, slipped the GHB, and then taken her out under some "you look like you might be sick" pretense.
I don't know if she'd been out much since having the baby, but 9 months w/ no alcohol made my tolerance super low, so I'm guessing same for her. She might have let someone escort her to the back, or the bathroom because she thought she was drunk when it was really just GHB. (Or the other way - she might have overestimated her tolerance and been tipsy even w/o drugs)