After the UK leaves the EU, EU citizens’ resident status in the UK will be dependent on how long they have lived in the UK and whether they arrived by the end of the implementation period on 31 December 2020, following our exit from the EU.
In this particular scenario, the EU national will continue to have the right to exercise her free movement rights to live and work in the UK up to the end of the implementation period on 31 December 2020. As the EU national has been living continuously in the UK for over 5 years (without an absence of more than 2 years after that 5 years), she also immediately qualifies for settled status, subject to criminality and security checks.
Therefore, she can apply for settled status under the new UK scheme once it is open. Once she has received her status she does not need to do anything further after the UK leaves the EU. She will have successfully obtained her immigration status in UK law.
She doesn’t have to immediately apply for settled status. She can remain in the UK after exit and continue her activities during the implementation period. However, she must apply to the Home Office by 30 June 2021 for permission to stay if she intends to carry on living in the UK afterward.