I'd like to know first: if they don't remember, how do they know that they were sexually abused?
If they suspect they were, then there is a figment of memory playing there.
I agree with dutch84 that hypnosis, if performed by a serious practitioner, can bring memories back.
If the person wished to try EMDR (Eye Movement Desensitization and Reprocessing: there is a hub that I wrote about the subject), then the mental image to sustain during the eye movement should be precisely that feeling that they have been abused, no matter how foggy it is. New material is going to surface pretty quickly, and can be treated with EMDR that breaks through the walls of isolation that encapsulate the traumatic memories, bringing knew perspectives and relief in a very short time.
R. Lorraine, I hope this helps.