It wouldn't be that surprising if the guy turned out to be cheating because, after all, this
It's kind of hard to lose something so strongly attached to your cell phone, but the guy is missing his face so he probably misplaces a lot of things.
I take LifeCharacter's stance on the protagonist's reaction though; it was just her immediate, not-well-thought-out emotional reaction. If it turns out it was just the guy's honest mistake, I think lil' sis has shown enough awareness/self-reflection that she wouldn't stay all blown up about it.
I think it's pretty valid if authors (yuri or otherwise) present guys as jackasses in teenage relationships in general though, because -- let's be honest -- the highschool dating scene is pretty damned shallow. I take that as a default stance for both guys and girls, the whole population. The main idea is, while it is nice to occasionally see a genuinely nice/decent guy in yuri, I don't mind at all the general portrayals of them 'cause most of the time they could actually be called pretty realistic.