I think you were right to give them a warning about being 3 stores up and
leaving Emma alone on the balcony. Having her leashed would be just as bad
because she could slip & hang herself with it.

You have made your concerns known, I would just trust them to hear the
warning & keep it in mind while they are there. Since your already said it
and they have heard it, I'd try to not worry anymore about it. (easier said then done,I know)