Really sorry for Tarumi. But she "betrayed" Shima through separation in middle school. Not imaginable for dog-girl Adachi who would turn the world upside-down just to stay next to her Shimamura.
I really wouldn't go as far as to say "betrayed", I know quite well how easy it is to drift apart at that age when you're no longer always together in class... I doubt Adachi would have fared any better, considering how long it took her to get back to talking with Shima even in the same class just because the other trio of girls started hanging out with Shima - and this already in high school. Had they been in separate classes as well, she would likely have disappeared into good old second floor of the gym, just as she almost already did, rarely to be seen even by her classmates again, unless Shima had found it in her to make a conscious effort of connecting with her once more.