You cannot read Amazon files on a non-Amazon device/app if the file has DRM.
And you cannot read non-Amazon files on an Amazon device/app if the file has DRM.
Depending on the non-Amzon device, you
might be able to exchange the files between device brands.
...
Basically, if you don't want to use calibre plus the DeDRM tools, your only option is a smartphone/tablet (possibly the E-Ink tablets by Onyx et al.)
Suggestion: convert the Amazon-purchased books for your wife, and get a Kobo since it plays nicely with the EPUB-only non-US library scene.