That bordering is not bordering if he is not told/communicated to. She is running covert contracts and mind games all over. It is similar to those games where the woman takes off on her own. Sed lyf for protag.
Again I don't think Rina has crossed any boundaries the MC has hard set. I think there's clear encouragment from MC to engage sexually with other people and she's doing so knowing it's what he wants. Now if he makes it clear he's getting uncomfortable with certain things and she still does it, then it crosses into cheating, but so far the only thing MC is worried about is the images sent to Hiroshi, but he was the one to push her into being sexual with him in the first place and hasn't expressed any discomfort with anything so far.
I think that what he meant is that this "consent" and "clear encouragment" may not be fruit of MC's will alone! Rina showed, since the beggining of the story, to be very perceptive. She already knew of MC's fetish even before he found out it was indeed a fetish. That's the reason she wasn't surprised in that laptop scene!
She's also good at manipulating things, those "mind games" Abra said. She keeps insinuating things, which have the effect of subliminal suggestion for people with a certain set of triggers, like the MC. For example, after the beach cabbin scene, when MC aborded the subject of shamelessly fucking in a practically public place, she asks "Were you bothered because you thought we'd do unprotected"! Knowing his condition, she knew that, sooner or later, he'd "ask her to do it". All she had to do was wait, and that happened in the end. She've being doing it since day one!
So, no, she didn't "crossed the border", she's skillfully manipulating and pushing the as she pleases, and worst, convincing her husband that this was "his idea", "his will", and she, being a "good wife", would gladly do "as he wished"! Rina is anything, but stupid, and is not even closely that "inocent" as she appears to be!
And even worst, being so perceptive, it's impossible NOT NOTICING the constant state of anxiety and disconfort the MC is in, since all this play began. Even if he didn't tell her, it's pretty evident, if you know where to look.
And, despite this, she chooses to continue to play the "mind tricks", pushing the situation further and further, instead of sitting down and have an honest and clear talk about the state of things. That's what a REAL good wife, who worries and care for her husband, would do! However, she looks more worried about sending nudes and sex pictures to provoke old perverts than with the health of her own marriage!
So, yes, what she do is not cheating. That's worst that cheating, because cheaters are clear about their intents. What she do is subterfuge, she "cheats" without needing to worry about hiding, and convinces her husband it was HIS IDEA!