you need to figure out which of the two top cases you have on your 2011...the difference is in the arrangement of the little plastic strut that holds the key to the top case. pop one off if you can (any letter key should pop off carefully from the lower left corner) and take a picture and i can tell you which one you need. your nearest AASP should be able to order you one and ship it to you...i don't believe it'll fall under the category of parts that can't be sold to an end user.
your wifes machine might use the same struts or it might not...take a pic of it too if you can...technically the keycap kits aren't sold for her machine but you can probably use one of the ones intended for a later model laptop.
just get in there as close as you can and take pics of the white strut with enough detail i can see.