Cornell is pretty isolated. I knew people in grad school who went to Ithaca College which is proximal and they said it is beautiful, isolated, and snows a ton in the winter (but probably not that different than Edmonton
). Higher than average suicide rate.
Oxford probably has the best resume fodder, and that's a big part of what a postdoc is about. But it really is a time to get a bunch of research done, publish, and make connections. A 6-month postdoc doesn't really give you enough time to do the above. Typically postdocs are 2-3 years these days, though at med schools maybe they are shorter.
Is she going to be away from her husband during the postdoc? While I know people who have successfully done that, it didn't work for me or others I know. Depends on the details.