Some women do get saggier from weight gain and then loss, and some do from breastfeeding, but mostly I think it has to do more with the shape of your boobs and how well you fit your bras. I suspect her boobs were a shape more prone to sagging to begin with.
If Lily’s been wearing bras that don’t fit her or suit her boob shape, and never actually scooping all her boob tissue in and rearranging, then she won’t be getting any decent support, so yeah, by age 30 (with an added weight gain and subsequent minor loss added in and a lack of exercise to keep her chest muscles firm), it makes sense she’s very saggy.