Base Camp (3) (Bill Walker's dialogue: Snow Line (8))
Overall, the prose in Major Barbara is pretty readable and easy to parse—it's not like you'll need to keep a dictionary in hand the entire time or anything like that—and aside from some historical/philosophical references, it's pretty accessible in terms of content. Nothing you guys can't handle here.
That said, we want to tack on a separate rating/warning about Bill Walker's dialogue, which is written in dialect. It probably wouldn't be the easiest to wade through if you were seeing the play performed—it seems he has an accent so thick you could cut it with a knife—but seeing the dialect written out phonetically is even more tough. So allow yourself extra time and an energy bar for that hike. Other than that, though, it's clear skies and largely flat land.