Ukraine should strip her of her passport and bar her from ever setting foot in the country.

To become a us citizen you have to renounce your other citizenships. So unless she later became a Ukraine citizen again, she no longer is.

Edit: It appears I am wrong about this. I’ve heard it several times before though.

To address your edit, some other countries only allow single citizenship. So if they get US citizenship, they are required to renounce their previous citizenship by their former government.