You are right. She was stupid to keep those letters, and they do prove that she was living with him while dreaming of another man.
However, never admit to an affair. Even if they have pictures , say: That sure does look like me, but it aint me.
Never admit to it. Always leave them some doubt.Deny, deny.
Sometimes they are looking to give you that way out. Once you make an admission, you are hung.
I've always found that the lying can be worst then the affair. I truly hate liars.
That may work on women. Men, not so much, IMO