Hi and welcome.
Since you've made your way over here to the dissociative forum I am going to guess that you suspect some type of dissociation might be involved. Can I ask what your partner's therapist has said about this particular topic? Your description here does give off a lot of clues about a dissociative disorder (it reads like a textbook listing off things that rule out common non-dissociative causes of amnesiac blackouts), so I am wondering what the therapist has said about any dissociative symptoms... has she (or he) just ignored them or glossed over them?
Not every therapist "believes" in dissociative disorders. They may well be written about in the diagnostic bible but not everyone believes in the bible. Some therapists refuse to even entertain such ideas.