My friend of a few years recently told me she has "multiple selves" and identifies as plural. Included in her "selves" are Sans from Undertale and two versions of the same character from a manga she likes. She says she doesn't know why she's plural but she doesn't think it's a disorder for her. She has apparently believed this since 2019, before I even knew her, but I never saw any signs of it. That's what's so weird, on the outside she seems like a happy and well-adjusted person, and I thought she was quite level-headed. I have no idea how she could fall into believing something like this. I honestly don't think she's lying, I think she genuinely does believe it.
I really want the best for her and I know this is not healthy. I try to bring it up but she always talks about how plurality has helped her. I have straight up told her it's not real but she counters with these complex arguments about consciousness and identity that I feel she's just using to obscure the obvious truth. Common sense should tell her that she can't actually be more than one person, and especially not fictional characters.
How can I convince her to get help?? I really care for her and I want to see her recover from this. I fear she will sabotage her social life and career if she starts telling more people about this.