The moment he assumed she worked as a waitress or whatever, you could tell that it was obviously not going to be something so...simple, especially in Hentai.
He seems like a decent guy though, like he genuinely wanted to stop her from doing that kind of thing, but I guess he just can't win against temptation. And in the end...He's just become one of her patrons, too. Rather unfortunate that he didn't bring out the "Make me your only customer" or something along those lines.
Once More, I don't see so much 'Femdom" as Mio-chan being (perhaps relatively speaking) an "Assertive Girl" ... but she carried it off quite charmingly.
Hiroya-kun tried his best to maintain the high moral ground, ... but in the face of "an offer he couldn't refuse", I for one won't blame him for succumbing (under breath, the Lucky Bastard).
In The End, perhaps a sleeping beast within Hiroya has been *cough-cough* "awakened", and Mio indicates in no uncertain terms that she's not entirely opposed to "one more time". Whether it remains "strictly business" or develops into *a-hem* something more, there can be only one way to find out.