She would do it just to occasionally look like she's a devoted servant of Andraste... except Bethe likes Desi, and she isn't one to talk when it comes to shady backgrounds and big mistakes. "He who casts the first stone..." and all of that. But HOPEFULLY by then they've have gone through several of those death traps together so there might just be some pillow thwapping. The secret coming out might actually spur a very serious conversation though.
And LOL YES. This totally has to happen. I'm sure they can find a giant free rolling boulder somewhere. I just hope none of them are wearing hats.