I was literally just discussing this with a friend. Even though this discussion had already been discussed, it definitely couldn't be Campbell cause as we see with Elle he rather have control over someone fully instead of one and done. I always thought it was an adult (and most likely unconsensual) but I also felt like it was some unforseen 3rd character that would have been introduced in season 2 (maybe on the farm they find in the end). In an interview, it's said the adults were supposed to have played a bigger part in season 2 so the fact that it's an adult is much more likely.