Yea I agree. Doesn't seem to make much sense that everything lead up to that is going to be packed in one episode. Makes even less sense if that scene happens regardless of whether you're on Jill's path. If Jill's just a friend then she's basically going with you to the hospital for your former girlfriend's mom who she has no real connection to, can't do much to help at this point (even with money, a scene like "the operation is going to be expensive" isn't going to happen at the initial outset), barely *hangs out with the MC outside of her route, barely interacts with the rest of the main cast, and at that point has probably only exchanged introductions with Zoey. What I'm saying is "Please, don't be about Zoey. I like the character but I just don't care." Lol.
Also, I don't think she's that selfish for someone her age. What would Zoey do? Put her life on hold just to sit around at home and comfort her mom? For how long? It's not like her mom's struggling financially or looks like she needs any kind of help. She also has her husband? boyfriend?(can't be bothered to remember). She has her own life to live and her grandma dying is shown to be the catalyst for her short story.
"Thinning" in this case would indicate a negative connotation meaning she looks "weaker" or "unhealthy". Usually shock or concern will accompany it. "Slimmer" would have a more healthy connotation. Still, it's not like it's a clear indication of anything dire in someone's eyes but is often used as cliche foreshadowing, especially if it's only "a bit thinner"which could mean anything or nothing at all. In fact, it seems more like it shows that Zoey cares about her mom and shows concern over well-being. Otherwise, she wouldn't have even noticed.