A lot depends upon what glass you have for each. If you have a good 50mm and say a great 25mm with a bright line finder for her I'd say Leica. If you have a kit for the Nikon wroth say a 28/2.8 AIs, 85/1.8, 300/4.5 ED-IF then that might be my choice.
Depends on your style, what you want to shoot, and what you got. Both cameras rock and do not need backup. Either kit would serve you well but I'm not sure what glass you have. I would be fine wroth either, not sure about others.
This is a bit like comparing Apples n' Oranges; rangefinder Leica vs the SLR Nikon.
Budget, and personal preferences aside, if you're unfamiliar to either camera,
then neither would be my first choice for you.
Maybe a camera in the point n' shoot category would suit you better for a trip,
which could tip the favor to the rangefinder Leica, for the purpose of answering your question.