She mentions that she's a lightweight who can't handle alcohol, and then proves this by passing out after a single drink.
Her nightmare that sends her running into the MC's arms shows a younger version of her drinking, with an indistinct male form in the background. She's a terrified shut-in who can't handle being seen by people.
It's pretty unambiguous what happened to her, even before the old people started gossiping.
She was hanging out with a bunch of older boys from college and tried drinking (possibly for the first time) she passed out and was sexually assaulted.
The fucked up part is that the old people (her parents or neighbours? It's unclear) put blame on her for being careless and say that she's 'broken' now, which is a very Japanese way of approaching things. Blaming the victim.
And they specifically mention "It was so bad that arrests were made."
Which implies that there's a kind of gangrape that isn't bad enough that people go to jail for it.
Also, her changing her hair back in the last chapter is a little...
It kind of smacks of "He took good care of her and she eventually healed and went back to how she used to be." which... I mean I get it, and she does look better like that, but it kind of implies that her experience actually broke her and now she's finally been put back together again.
Like everything she did and accomplished until now didn't count because she wasn't really herself or something.
I dunno, her being healthy and happy but with the pink hair would have maybe been more fitting.