As far as France is concerned, the person is a French citizen, nothing else matters. As far as the US is concerned, the person is a US citizen, nothing else matters. So they will both likely be ready to evacuate the person, and the practical result will be that the person is allowed to choose.

I mean: I don’t think they can force anyone to evacuate. If the person says “no, France, I’m not getting on your plane, I’m waiting for the American one”, then what can anyone do about it?