Stories like this get me. If his (hopefully "ex" soon) girlfriend was unsatisfied with the relationship/sex she should've said so, but I guess it's easier to go behind your bf's/gf's back and cheat on them instead of talking out your issues with the person you apparently love. /s
He really should just dump his girlfriend already and get together with his lovely Kouhai who clearly cares for him.
I'm not sure why he hasn't already broken up with her already, honestly. The perfect girl is right in front of you, offering to pull you back up...At least he attempted to take responsibility, but I think what she really wants to hear is not so much that you'll take responsibility after having sex with her, but a certain 3 word sentence with the word 'love' in the middle...That or just asking her out properly after breaking up with a certain cheating girlfriend.
After he came to school again he heard that his girlfriend fell of the balustrade. I mean, look at how she was making out with the other guy on that thing... it was bound to happen...