Everyone dissing out their relationship troubles as "drama", but let's be fair here, remember how you and your school friends were during that age and tell me these sorts of problems didn't happen.
I didn't have a girlfriend through high school, but I had my fair share of what people would call drama with my female friends back then, let alone what I observed from a distance what they did to each other.
Back to the plot, though, Mone could be one of these:
1. Actually be kind and worry about her friend, even though she badmouths her to her other friends, as people often do among those they're close to;
2. Actually be into Oogami and be trying to get close to her by taking her away from her boyfriend;
3. Be nice to Oogami when they're alone and badmouth her behind her back to fit in with the group.
Whatever it is, Yaginuma could just touch her and make her spill the beans, but he hasn't thought of doing that whatsoever. His lack of social skills prevent him from taking the step he needs to (up until now he couldn't even tell his mother what his plans for the future were without external interference), so it makes sense he'd instead try to get Oogami away from Memo without telling the truth - because he's an idiot, as most teenagers are.