Anyone who is claimed as a dependent is not eligible for the stimulus.
Technically, the stimulus is an advance payment on a tax credit for the year 2020. So it would be possible to not claim the adult in 2020 and have them file a tax return then and claim the credit.
You could attempt to amend your return this year, and have the dependent file (or amend) her own return. I frankly do not know whether there is time for this or not as a typical amended return takes 6 weeks before it is processed.