I Only Intended to Peek Through my Girlfriend's Phone

With his cute girlfriend asleep on the floor, and her phone lying right next to her, his curiosity gets the better of our protagonist and he starts looking through her photos. Each picture of his childhood friend reminds him of her soft, chubby body, her enormous boobs, and the weird faces that only he gets to see. He's head over heels in love with her, but as he looks deeper, he's seeing some more racy photos that have been sent to someone named "Yuuya-san." Who is this guy and why is she sending him nude photos?