And in case you're not sure how to unlink from Amazon, it's actually really easy. Make sure you're logged into your own Amazon account, then visit http://amazon.com/roku
and click the unregister link that should show near the top of the page.
In case you don't know, you can remove the channel by either pressing the * button when on the home screen (if you have that button on your remote) or by finding it in the My Channels row in the Channel Store.
Relink to your wife's account (assuming, as Gilgamesh pointed out, that it is a paid Prime account), and you're all set!