Basically, periods where they just black out for a while and then have little recollection of what happened. Its obvious enough that even others unfamiliar with mental health have commented on it. It also seems to come with a change in personality, not as in pop-culture multiple personalities, but more like she forgets key experiences, or certain feelings. For example, being out and having a good day, then out of nowhere, being despondent and while she might be able to robotically list off the events of the day, she can't conjure any of the related emotions. Before she learned that it wasn't just a normal chronic depression thing, she described it as feeling like "the sunken place" from Get Out, watching herself doing stuff. Watching it happen as a friend, its like she's possessed. An overly-religious person in her life literally tried to perform an exorcism on her (which would be really funny, if it weren't so sad). Even in terms of background, she has more than enough trauma to raise a ton of red flags for things like BPD, DID, PTSD, CPTSD, etc. but all of that has largely been ignored. Like I said, pretty obvious and textbook case for something like that, but its mostly just been dismissed, usually as ADHD or bipolar.

Edit: and to be clear, all of these symptoms have been present for decades, well before either of us was much aware of mental health as a topic, nonetheless dissociative disorders and their modern romanticization.