All you can do is message the guest and remind them of the regulations that are currently in place and that they would need to isolate for the duration of their stay if their country is not on the green list when their stay occurs .
@Nicky3 No one knows when restrictions will be lifted or reinstated, in most parts of the world. It depends on infection rates, new variants, etc. None of us have a crystal ball and all we can do is take it as it comes.