This was mentioned in the episode thread, but I thought a reply would be better here:
The thumb tucked into the palm can't be a coincidence: she's signing B/Bitch. Margie, in her two-tone jacket, can be seen behind the tree hurrying to catch up with Luke, who stepped away to the right. I'm not conspiracy-minded, I don't think the bad blood was staged by production, so if we take the editing at face value then Luke didn't see it go down (Margie told him in the taxi). It also leaves the interesting question of where Jen got her word skills.