Éponine stares at Cosette throughout the interview with the charitable gentleman and his daughter. Does she recognize her as her father recognizes Valjean? Or is she merely envious of someone her age who is living comfortably, even lavishly by comparison?
She definitely already has a crush on Marius, based on the next scene, and she’s utterly dejected when he asks her to find the girl’s address.
The book never addresses Éponine’s reaction to learning that the well-to-do woman Marius pines for was her childhood servant — only her parents’ reactions. (Her father mainly nurses a grudge over not having managed to extract more money, but her mother is infuriated at the reversal of fortune that Cosette so well-off while her daughters have nothing but rags.)
Éponine isn’t much of a viewpoint character to begin with, which I think is a missed opportunity. We mostly see her from the outside as she supports other peoples’ designs. She gets to make some fascinating choices (my favorite is standing up to Patron-Minette), but by the time things get really interesting, psychologically speaking, she drops offstage to work behind the scenes.