Basically, title
Do not try to become her manager or parent or psychologist or coach. It will negatively impact your relationship.
https://www.getinflow.io/post/adhd-authority-issues-defiance for some ideas on why.
If you have a problem with how she is behaving, you will not be able to change it through external influence. If she really has ADHD (emphasis on disorder where it is negatively and significantly impacting her life AND is diagnosed) and is unmedicated, the single best thing she can do is get medicated, and that’s her choice. If medication isn’t working, she should talk with her provider about it.
If she requests help, feel free to provide it. Feel free to ask her (and not internet strangers) if there’s something you can do to help her as she’ll know what works and what doesn’t as ADHD isn’t a monolithic diagnosis and what works for one person might drive another up the wall.
Sorry if this seems a bit negative, but I was in school when all the Where There’s a Will There’s an A and all the techniques in the world did not make a difference to people that can’t utilize them, but they can frustrate and shame people.