OMG op you must be frantic. I have a dd with ASD, formerly with ARFID, and I completely understand how helpless you must feel and how tempting it might be to respond with trying to gain control over the situation whereas I think the reverse would probably be better strategically speaking.
Calm and gentle discussion.
Maintain the relationship and keep communication open.
Use the ASD brain to your advantage and provide her with lots of research and sources of info after trying to get her to agree that if she is going ahead, she needs to examine the issue from all sides. So approach it from the perspective of, I can see you want this, so let’s try and protect you and make sure you get the best treatment available pov.
I would engage in some philosophical discussion about different perspectives of beauty from the point of view of different cultures and historical perspectives?
And I would try showing her the NHS website (there’s a bit about plastic surgery) emphasising the importance of going to a surgeon who is reputable explaining how some surgeons just do it to make money and don’t care about the patient.
Then I would probably try and find other information from other reputable sources about how things can go wrong.
There must be a Facebook group?
Do you have any doctor or nurse friends who could advise?
I would probably try and find some on-line testimony from teens and young people about how they regret doing it. Tik-tok?
Find out where she is accessing her information from.
Try and show her a series of stills of a favourite celebrity where it went wrong in rl?
Michael Jackson springs to mind but it needs to be someone she relates to. Explain about how, if you unbalance the face by surgical means, you often end up having to go down the route of more and surgery.
I think you can be supportive whilst very clearly not supporting it ifyswim. Would she let you come to an appointment, if you promise not to say anything?
But before it gets to that stage I think I would try and negotiate a truce with her and say that although she is over eighteen and it’s her body and face to do with what she wants you love her so much and want to keep her safe; if she is going to have someone literally cut in to her face with a knife, as it’s such a significant step, could she hold back and wait and discuss it again in a year? Maybe phrase that differently!!
Another tactic, ask her to find twelve friends and acquaintances of her age and ask them if they thinks she needs plastic surgery?
You don’t want to risk alienating her and you obviously don’t want to use all of the above tactics all at once but maybe one or two might get through to her?