the way i see the sex scene was a dream/imagination coming from the friend or happening in the gyru guy dream.
we know she got some powers, invading dreams, we know succubus can knock people out as we saw the other girl do in the past, creating illusions and what not, i do believe that in that night she did "something" to the gyru guy, what i would like to believe is that whatever it was it was not sex.
the point that makes me huff copious amount of copium comes from the condom dialogue, Jp dialogue can have multiple meanings depending on many factors, using a translator app will give questionable translations sometimes, so i am hopping that it is a translation error, because her going basically "yeah nut in me as much as you like no condom" feels bad man

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another detail i like to point out is the fireworks, in japan missing a festival and fireworks with your friends, family or loved ones is seen as a really bad betrayal, one could interpret the fact that she gave up on going out with the friends and decided to stay and suck dick as her giving up on something, the friends, the childhood friend or even her dignity as she more or less "lost" to sex/desire, because at this point she had already succeeded in her mission and that last interaction was just her indulging herself.
i do agree on the point that she is the predator adapting to her prey, the last page is her back at her "normal days" but it doesn't feel good because we know what might have happened and what did happened.