my left foot is a half sized smaller than my right so i feel for your wife. tbh what ive finally started doing is finding a good pair of boots and buying two different sizes. i shit you not this was my last resort but it works the best. now i know not everyone has the luxury of shop hookups that make it possible to get two pairs of boots for price of one ( btw my hookup is friend based not because im super awesome pro wanna be dude lol ) so you can also check the last years bins and online retailers. id only buy online after trying the boot on at a shop tho and not make a blind purchase.
as far as heel lift.. alot of time with new folks they just dont understand the mechanics of properly putting the boot on. have her standing with pressure/weight on the heel of the boot like she will be when shes riding then tighten the inner liner. thats what works for me. trial and error with tightness of the liner and laces will eventually lead her to the best amount of pressure and tightening to help prevent heel lift.