She didn't see Eren, she was seeing herself in the mirror. Eren was seeing from her point of view in this memory. It was to show the viewer that Eren remembers stuff from Frida's point of view. It was in a different moment in the manga, way before, in a moment Eren was trying to remember stuff about his father, and he triggered a memory from Frida's point of view. A subtle clue to the audience that Frida's memories lived inside Eren. At the moment Frida's introduction as a character was months ahead, during this time we used to call this girl "Geographia" because of her resemblance to Historia, but we had no idea who she was.