Ah, Koyuki. You've got a hot girl in your bed and you snuggle up with your Telecaster. What fun.
And hey, your mom likes her. That's a plus.
I'm actually writing this about four hours after watching the episode, since I had to go out, so my memory's kinda fuzzy, but the scene of Ryuusuke and Maho breaking into Koyuki's house was just pure win, from "Hey, I thought you didn't want to wake your parents?" to "Here's some protection."
And I reckon I can see where Koyuki stands on this love triangle now that he knows it exists. I also reckon that he's not going to be stupid about it this time and deal with it sensibly and tactfully.
But who gives a crap about that? JAPANESE GLASTONBURY!!!