Coping with fears of bad things from the past

A person I know well recently realised that they did something bad in their past. When they were a younger teen, they seldom touched inappropriately their sibling who is 4.5 younger than them. Everytime they were told it's bad, they stopped doing it for some time, but they didn't actually realised that was very bad behaviour until they got smarter and more responsible. Now they'd never do it again. But when the thoughts come back, they hit them so hard. They feel very guilty, ashamed and sometimes they can't even get out of their bed and do things they like or dream about their future, because they are feared it'll become known. They're ready to apologise for it, but they are certain it will not help them. Now they know it was bad, and they sometimes think death will help them. The sibling probably doesn't feel hurt though or uncomfortable, but the person can't know exactly.
Is there something I could do for this person? What should I tell them? Are their really that horrible as they feel? And should they be really that scared? Will people even care?
Thank you very much for responses.