How about telling him what he's already done to her?

Evelyn didn't want to stop while she was just figuring out the trick. Again, the clever slut thought about what John might want. She realized what she could say, and it was so simple. She just had to explain how she got where she was now. She got an idea, and started spitting on her tits to lube them up. Then she slid John's cock in-between them and continued talking to him as she fucked her tits on his dick.

"You do realize what just happened, right? I already wanted your dick in my pussy, that wasn't new. I already wanted your dick in my ass, that was old news too. I have dreamt of your cock pumping cum into every hole I have... but I've been treating it like a spout. Just pump it enough times and out comes the delicious nectar to fill my holes and my belly. But that stopped working... and, more importantly, you have literally starved me to the point of desperation for your cum. " Evelyn looked up at him with doe eyes, and then tilted her head down to lick his cock head between her tits as she continued to bounce up and down.

"You may have been trying to make good business decisions, but I think... I think I'm done with that. I'm too **** for your cum to have the patience for professionalism. I need as much of it as I possibly can, all the time. And the only way to do that is to be your slut. Your whore. To find the nastiest shit I can do, and then do it before you even know you want it. I need to be the hottest little fuckpig you've ever seen, so that when you see me you will feed me the only food I'm allowed. I am starving to be your slut. I will eat your ass. I will fist my pussy, ass, and mouth. I will put those thumbtacks you bought directly on my clit, even.

"I need to take our relationship to the next level. I don't want to be your employee anymore. I need to be your whore. Please, let me be your disgusting slut. Let me be your cum dumpster, your fuckhole, your pain slut, your toilet. I will be anything and everything you can possibly imagine, because above all else I'm ****. I'm **** for your cum, spunk, jizz - whatever. Make your cock explode and cover me in your juices. " John looked at the wild eyes on the slut in front of him, and came a second time.

John was shocked. Evelyn had moved so much faster than he could have imagined she would. Somehow the combination of sexual stimuli, masturbating mind-fuckery, sleep deprivation, and starvation resulted in an incredibly powerful drive to get as much cum as she could. Like last time, she put on a show for him. She didn't just swallow the cum on her tits... she licked it off slowly, cooing and seeming to savor each strand. She was putting on a show, and trying to turn him on again for another round.

But Evelyn couldn't help but notice - this was even worse than the last batch. The mix between tasting disgusting sewage and having to act like it was delicious was... jarring to her.

Once John had collected his thoughts, he realized he needed one more thing.

"Prove it," he said.