I'm not convinced these are orgasms... are you sure it couldn't be some sort of sleeping disorder? Like sleep apnea or some other breathing disturbance?
Since she can't recall sex dreams, she doesn't remember orgasms?, and it happens so often, it just doesn't seem likely that they're orgasms. I mean, if she woke up and her vulva was swollen or something, then maybe I guess.
But anytime I've had a sex dream (and orgasm in my sleep) I definitely remember it.