Start with 1 John 1:9, if we confess our sins he is faithful and just to forgive us, and Romans 8:1, which says there is now no condemnation for those in Christ Jesus. Psalm 103:12 about God removing our sins as far as the east is from the west, and Isaiah 43:25 where he says he remembers our sins no more.
What helped me with these is realizing God already settled the matter. I went through a time of carrying guilt I couldn’t shake, and someone pointed out that refusing to accept forgiveness is like saying God’s forgiveness isn’t enough.
You said this isn’t their fault. If that’s true, they may need freedom from false guilt more than forgiveness. I’d point them to Psalm 34:18, the Lord is near to the brokenhearted, and Romans 8:38-39, nothing can separate them from God’s love.
Being there probably matters as much as whatever verse you pick.