My partner has DID, and I've done a ton of research into it as a result. This story sounds extremely plausible to me.

I've read multiple case studies where people with multiple personalities will get out of whatever situation was causing the disassociation, and over time some of the personalities will vanish / die off. There was also a very extreme case I read about where the fractured personalities managed to coalesce into a new whole, but it was a different personality than the original. Basically a fully formed identity that was suddenly living the life of someone they didn't identify as, and whose memories they couldn't really recall.

Even in my partner's much less severe case of DID, the less prominent personalities will sometimes go dormant for months at a time. Haven't had any of them disappear fully yet, but it's at least theoretically possible from what I understand.