I'm also in agreement that I always choose the backing last, however, I'm usually not very concerned with the backing closely coordinating with the top. As DogHOuseMom mentioned, I will sometimes use one of the top fabrics.
When I'm on vacation, I don't want to tax my brain too much. I would choose patterns I was familiar with, or maybe do the same pattern in a couple different color ways. If transportation is available, I would also consider fabric shopping while down there. A quilt top with fabric purchased on vacation is a great souvenir. She could also look for backing fabric!