I read it as well, and also listened to his podcast (which is where I learned about "TEAM-CBT", his supposedly highly effective new approach). The concepts and arguments in the book (and podcast) make total sense to me, but the _way_ it is presented is sounds more like snake-oil marketing than science, which is why I am interested in learning about its effectiveness on severe forms of depression from independent sources.
It helped me a lot, in particular in dealing with thoughts something like what the OP's partner apparently had. It has great strategies for first recognizing, and then dealing with such thoughts. They're not "just feel better" ideas, they're specific things to do which help.
Highly recommended.