Good for her.
It might have just been a big mistake. Having once been in my 20s, I can tell you that you don't automatically become a perfect adult once you hit 21. Sometimes you say stupid things. This could have been one of them. The original invite could easily have been OK, and when his girlfriend couldn't make it he might have just gotten a little confused, thinking it would be no big deal. Since he said he'd already realized it wasn't a good idea sounds like he had done some thinking in the meantime.
So losing all respect for him seems as big a jump as the poster talking about how he's "grooming" her...especially since he's a young teacher, he probably just got a little mixed up momentarily.
The fact that you guys were able to figure out that she needed to not go says good things about your college years and beyond. College is when it can be really confusing...I actually dropped a course that I needed extra help in, because the teacher had a major reputation for "wink wink nudge nudge" types of after-hours help... I couldn't get the guts to get the help while standing up to the guy IF he tried anything.