

Well, there is partial truth to this. Diagnosing especially preteens with mental health conditions almost always needs an asterisk, their brains are still heavily developing.
Remember that we know of no reliable genetic / physiological markers for ADHD, so when doctors diagnose it, they’re really just saying that the amount of symptoms seems to be severe enough. Some kids are just naturally more energetic, intelligent, etc. and may appear like they have ADHD, but when they develop into adults that might not actually be the case.
If anything, it’s more like 1/3 of kids were misdiagnosed as having ADHD, so they “grew” out of it because they didn’t really have it. It’s a lot less likely for an adult to be misdiagnosed because their brains aren’t changing.
You’re right that this might look like BPD, but that’s also a huge conclusion to draw from only a couple sentences. It’s extremely rare to diagnose BPD before age 18 because it has too much overlap with just… the nature of moody teens and puberty.
OP, it might be useful to research BPD but moreso than anything, just give her time and make sure she knows you’re there if she needs you. Give her reassurance and try asking her about why she thinks people hate her, and maybe try working through it with her (try bringing logic into it, but try to get her to come up with the logic rather than telling her directly).