I have no idea, but if you're using Chrome try this:
CTRL-SHIFT-N, it will open up an incognito window. This window does not remember any sessions or cookies and will function as a "fresh" experience.
If in IE 11 (and probably other versions of IE also), open up a new tab and on that main page is a link to "InPrivate" browsing, which I presume is the same as Chrome's incognito.
If the above solves your problem it sounds like Fitday isn't managing cookies properly and there is contamination between you and your wife's account, hence the problem.