I think it's fair to assume that although Nagatoro's two lady friends are also jerkasses, they had already caught on long ago that she has a serious thing for Senpai. They know that, given his personality (and her personality), that this couple-to-be will not properly progress unless they intervene and help them along, in their own, twisted fashion. Whether or not they knew that Senpai would end up at the festival, alone and wandering around like a lost puppy is debatable, but either way, their sending Nagatoro a picture with him in their clutches was purposeful. Way to go, winggals!😜
One thing I'm curious about, is on page 12, the lower left panel, when Yoshi says "Go back to the mountain!", what is that in reference to?