I'd recommend involving a lawyer at this point. I also wouldn't go to interview without a lawyer, this doesn't seem like a DIY case. You're looking at potential material misrepresentation and inadmissibility based on that.
That's great, but USCIS still needs to know whether she was ever arrested, detained, charged etc. Even expunged records need to be disclosed to USCIS.
I think she can write an affidavit explaining everything, if she doesn't and can't produce any papers.
One more thing to note. Just because everything was OK for DS-160 and K-1 doesn't mean USCIS won't make it issue later during I-485, I-751 and N-400. There are known cases when only at N-400 USCIS would find issue and claim that Green Card was given in error. Each stage of immigration is chance for USCIS to review entire immigration history.
So being open and upfront with USCIS is the key to securing trouble free future. It may mean immediate problems and waivers involved at this time around though.