I'm autistic for reference. It's a thing for people to "become more autistic" after their diagnosis. There are a bunch of different reasons for this but I think a lot of it is feeling like you don't have to mask anymore, which is great, but there's a period of not really knowing who you are or how you want to present yourself anymore. Most of us eventually find a balance between our true selves and our masked selves. Autistic or not there are things we all need to do to be a part of a functioning society and we can't just do, act, or say whatever we want.

I don't know how you should handle your friend but they're obviously going through some sort of identity crisis. You said yourself you think the diagnosis is accurate so it's not like they're making things up. I'm sure they'll figure things it out in time. You're allowed to have your own needs and boundaries though so do what you need to do while they figure it out. Maybe just make sure they understand you're not abandoning them.

They freed themselves from the mask, and they're trying to find the real them. It will pass, give it some time. I ended up rewinding my life back to teen interests when I realized that that was the last time I felt truly like myself, and allowed those interests to grow and integrate into my other adult interests. That whole process took about half a year

Since she recently got a diagnosis, I'm taking she's being regularly followed by a mental health professional? If so, to answer your question, I'd say ride it out and let the professionals do their thing, while maybe distancing yourself a bit since it's bothering you.

"dude, quit mimicking a downs person. You may not clue in on it, but it's uncool to do. Now, what were the dinner plans again?" Or whatever the convo was; just redirect back in once you've clearly made the point without dwelling on it.

Sooner is better.

You can try to ask her how she feels about her diagnosis, what symptoms does she feel suits the diagnosis best for her, did she feel like she had to hide parts of herself before and what parts, are there any symptoms that she doesn't have that are common with autism. Questions to figure out what she thinks the diagnosis means and poke a little into if she feels like she has to put up a front in some way, or if these are just tics she's been hiding and perhaps some natural regression when getting diagnosed.

You can also go for the more direct route, kinda prime her with "I know most autistic people prefer direct communication, can I speak honestly to you even if it may sound a bit harsh?", and then explain that you noticed she has changed her behaviour drastically since being diagnosed and you're worried that she's not being her authentic self but instead trying to fit into some stereotype, especiallywith actinglike she doesn'tunderstandthings that she used to understand.

Or just call attention to it in the moment (when you are alone!) if she's acting dumber than she is, like "you used to be able to solve/understand this, why do you not understand it now?" or "why are you doing that thing with your face? You've never done it before, is everything okay?" or "you're being quite loud right now, can you please adjust the volume?" or whatever the specific thing is that is grating.

You are their close friend, and your friend is going through a very tough time in their young life, as much of a core experience as a person can have, and they need love and guidance, not abandonment. Modeling that on both sides would be detrimental for life.

Talk to your friend, feel their feelings, and help them in their next steps. Perhaps that's already happening with her family. They've certainly noticed this odd behavior, too. As her friend, perhaps you should meet them, and help support whatever strategy they are following, or if there even is a strategy. They may be overwhelmed, too, and may welcome your assistance.

Or maybe not. Be prepared for that, too.

Be her friend, don't quit on her, and someday, she won't quit on you.

Man, as an Autistic person who has, how can I put this diplomatically, been perceived as having DS by those who are unaware? (I have a big head and the tendency to mumble and indulge in Echolalia) This is a whole ass vibe

In my experience there’s not much to be done, you can’t talk them out of these displayed behaviors cognitively if they didn’t come across these behaviors cognitively eg

*were they evaluated for DS in the past?

I had a fellow autistic schoolmate who was evaluated for schizophrenia before they were diagnosed as autistic, so they tended to interpret things in that lens, like if someone is having a meltdown, to an outsider does it present that much different if they’re have an overwhelming sensory moment, ‘hearing a voice’ or say, having a seizure?

*have they socially interacted with someone with DS?

I’ve seen it been super common for autistic peers to pick up ticks/phrases from others without even realizing it, even if the interaction was years ago

And probably the last step would be exposure, if there is a person with DS in your social setting who would be negatively effected by your friends actions, could you talk with their family/any supporting people that have about introducing your friend to them?

And if not, then their actions seem to mainly be stressing you, and maybe you should decide if your friendship with them is benefiting you both equally and should continue

Not to dump this in your lap or say that you have to be the one to do something or change, but in my experience you’re going to have more luck engineering a situation where they figure it out themselves, than changing their mind with an argument

Gather the courage to tell them what you're telling us in a polite way. In my mind, there's no other way around it.

A good friend will confront their bullshit in a loving way.

What kinds of behaviors do you want to reinforce? What do you want less of? Is it possible to get more of the good stuff and less of the bad stuff in your current context?

If yes, try it.

If no, I think this is a matter of acceptance. It’s really unfortunate.

Start acting like you have Downs Syndrome as well when you’re around her. See how she likes it.

Yeah time to move on