You ask a reasonable hypothetical, but I don't think it matches the circumstances here. If Yates felt that she could not in good faith enforce the order because it was unconstitutional, I think she would have been very clear about this in her memo: https://www.nytimes.com/interactive/2017/01/30/us/document-L...

Instead, while not ruling out unconstitutionality, she pointed to the "broader" responsibility she feels the leader of the Department of Justice should have. I thought she said quite clearly (for a lawyer) that although she was uncertain whether the order was legal, her refusal was because she judged the order to be unjust and unwise. Personally, I greatly respect her differentiation between "constitutional" and "right".