I really don't understand how anyone who has read the story up to this point would think the author would do an ntr arc.
We already had that with the poly dude going shopping for presents with meguro.
He ran away but then apologized for not trusting her enough to even let her talk.
Koga knows her x successfully tutored her before, and koga self admits he might be to biased to tutor her himself. He worked with the guy long enough to get to know him. He trust his ability and integrity, and thinks asking for this favor is the best thing he can do for his girlfriends future.
It's that simple.