What decision does Tama makes that jeopardizes her friendship with Subaru in the novel, The Ten Loves of Nishino?

Tama first meets Nishino through her best friend and roommate Subaru. Not long after Nishino and Subaru start seeing each other, Nishino begins dropping by the apartment when Subaru is not home. These visits lead to Tama's and Nishino's friendship. Over time, Tama becomes increasingly confused about who Nishino and Subaru are to each other, and who she is to Nishino. Therefore, when Nishino finally tells her that he wants to be with her, Tama agrees to have sex with him. This decision causes a fracture in her friendship with Subaru, and distances Tama from Nishino.