Resources for adults supporting a partner with childhood sexual trauma
Rape and sexual assault definitely does not only affect the assaulted, but also the friends and family as they can feel helpless, not knowing what is best to do to help and support the assaulted.
As I mentioned in the comments, support groups such as

RAINN (US)
Call 800 656 4673 or 800 656 HOPE
https://www.rainn.org and
Survivors Trust (UK)
Call 0808 801 0331
https://www.thesurvivorstrust.org
See also: https://www.thesurvivorstrust.org/national-helplines for other national helplines

provide support to survivor's partners, friends and family too.
The following books are recommended by Edinburgh Rape Crisis Centre

Allies in healing: when the person you love was sexually abused as a child, by Laura DavisISBN: 978-0060552992Laura Davis is also the author of the well known book for survivors called The Courage to Heal Workbook which is a companion to the main book The Courage to Heal she co-authored with Ellen Bass
Allies in Healing is available through Amazon as a Kindle book for a much cheaper price than a printed copy if that suits you.

Ghosts in the bedroom: A guide for partners of incest survivors, by Ken GraberISBN: 978-1558741164
Ken Graber draws from personal experience to show how partners can accept responsibility for their own issues, support the recovery of the incest or sexual abuse survivor, and work toward solving relationship problems together.