In one of the later scenes, Ava whispers something to the what I surmised to be a sex bot to cause her to cooperate in killing the mastermind. What did she say?
Alex Garland spoke to this exact question in an interview with David Onda at Xfinity.com. In short, the two machines; Ava and Kyoko converse in machine language in a way in which we mere humans couldn't possibly understand. The gist is that they collude to kill Nathan and escape the villa:
Garland: I can’t tell you the conversation she has with Ava. [laughs] I guess it’s along the lines of, “Let’s kill this guy.” There’s a sort of point embedded in that conversation actually, which is very obscure: When these A.I.s – if they turn up, if we ever get strong A.I.s – they might be like us in some regards, but they also won’t be like us. In fact, they’ll be very, very different. We won’t really be able to understand what it’s like to be them, and they won’t really understand what it’s like to be us. And the empathy that they will feel will probably primarily be with each other. And the empathy that we feel is primarily with ourselves. So it has to do with that. Actually, it’s literally beyond us, what they’re talking about. It’s their world. It’s their language.