It's really hard when our kids turn 18 and all of a sudden, they're responsible for their health, records, filings, etc. We moms have gone from organizing their records, managing their 504 plans and IEPs, to all of a sudden being told "I'm sorry, I'll need your child's permission to speak with you about that". It can be frustrating.
My daughter started her SSI application the week after she turned 18, and thankfully she was very willing. But she had to complete the forms and answer questions, not just me. Of course, i could assist with certain things but she had a large part in it.
It's also difficult for a teen or young adult to face the fact that they are, in fact, disabled. They may associate receiving SSI with certain negative stereotypes or cruel words ("welfare mom", etc) that aren't accurate.
But the truth is, you can't force her to apply since she's 18.
Oh, and if she should change her mind, I have a suggestion. Don't just submit her IEP and records. What will be the most helpful is a brief statement from a professional (doctor, psychiatrist, psychologist, etc) regarding employment. The statement should avoid complicated diagnostic terms that we may understand but that an intake rep at Social Security may not be familiar with. So here's a sample:
"[Name of person applying] is unable to be consistently employed due to the nature of her diagnosed disability. [Name] is not able to follow directions or complete tasks, and would present a safety risk to other employees. He/she has panic attacks and takes medication that makes clear thinking more difficult."
In other words, state why the person shouldn't be working, not what diagnoses she has, or how her schooling was affected. My daughter's letter said that she was a fall risk, and that her medications cause unpredictable sleepiness and fatigue. The letter referenced her frequent medical appointments and sicknesses. Then the doctor stated that in his professional opinion, my daughter would not be able to seek employment or safely perform tasks. It avoided referencing things like Inappropriate Sinus Tachycardia or Ehlers Danlos Syndrome, which of course she has (among other things), but those are not very ordinary diagnoses and would just muddle up the application.