He almost looked hurt. She didn't know what was happening. Wasn't he supposed to be proud of his handiwork? A potion on the lips was the only theory that made any sense. She didn't drink anything he could've slipped a potion in when they were together in Edinburgh. Neither did Alecto at the funeral. They must have taken the antidote to whatever potion they wanted to use on her and then transferred it to her skin using their lips.