He's not present. The scene with Pastel is not changed in any way. There is no special dialog relating to this. It just sets a flag which ensures that at a particular point in the path, the path is automatically failed.

As for why you fail this path if you kiss Pastel, I leave that to the detectives examining the mentality of the characters involved.

So it's been yearssss since I first posted this question, but I was recently thinking about this game again, and I think I might understand the answer.

Maybe this first thing was obvious to people, but my theory is that Grabiner was attempting to resurrect Violet in exchange for his own life. Whether or not it might have been successful, MC interrupted the ritual.

As Potsdam says at one point, MC is similar to Violet in a lot of ways. Spirited, bright, brave, a Wildseed. I forget the actual words used, but basically, as Grabiner gets to know MC, he realizes that in saving MC's life, he was saving someone very much like the girl he loved.

When MC asks whether she can still date, he explicitly says no, that her social life can hold off for a year. I think for him, it's a matter of respect, and also acknowledging the loss (Violet's life) that interrupting the ritual cost. And a bit of penance for MC, to teach her that the magical world is very dangerous, and messing with things she doesn't yet understand incurs Consequences. Even if it's just...having to be married to a professor for a year, and not being able to date.

By kissing Pastel, she shows that she isn't really treating it with the gravity that he expects, and also revealing herself to be less like Violet than he thought. Violet was willing to follow him into the Otherworld. MC, by kissing Pastel, shows that she can't even respect a contract, let alone show the constancy and loyalty of affection that Violet did.

So even if he didn't literally See the interaction, in the game world, doing that kind of thing means that the MC demonstrates traits he picks up on otherwise. Am I somewhere close?

That theory is very interesting, and likely exactly what was going on! Grabiner was honestly fascinating, and his storyline leaves so much up to the imagination