I wonder if Shimamura broke what's-her-face's heart, or if it was the other way around...
I got the impression the other girl was never that invested in their friendship, at least compared to shimamura. Guess we'll see though.
What's-her-face = Tarumi. I'm not gonna put spoiler bars on that because meeting her again just happened in the last manga release, since it finally got ahead of us in the LN. And I assume we will be following the standard "Adachi then Shimamura" chapter pattern, so we'll be meeting Tarumi next chapter since now it's Shimamura's turn.
Shimamura wasn't super into Tarumi in any special way, according to her flashback chapter. She was the little girl she met in kindergarten who liked shiny things, and they just "grew apart" by virtue of progressing in grade and getting stuck in different classrooms.
I have a feeling, though, that that casual indifference was not mutual, lol. Can't have a proper yuri story without a foil to create drama. Which to me is kinda disappointing, because I think Adachi already has a lot of socially awkward feelings on her plate, so much so that she's a constant flailing lesbian mess. Having someone show up she can legit be jealous over is gonna be a bit much for our little cinnamon roll... I forsee many, many more "WHADASHAA"s in her future....