I get that people are human and forget and that is totally understandable, maybe even about abuse (although that is such a difficult topic and therapists should at least make a note or something) but doesn't it bother anyone else that this is the reason?:

" he said he was still trying to figure out how he forgot, but he thinks it's maybe because it was so hard for him to hear the details because we are so close and ???"

Really? That is so weird and bizarre to me. I get that it could happen in a non-therapy relationship, but in therapy? It doesn't seem quite right to me.