I received a letter of apology from the OW, and in some ways it helped.
Prior to the affair being revealed, she and her DH had been our friends. I was not very close to her, but we had spent lots of time together and our DCs were friends.
Once the affair was exposed, she wrote to me to apologise. I didn't enjoy reading it at the time, mostly because I wanted to hate her, and this made it harder. But I did reply, just to say I accepted her apology but did not want any further contact.
Over the following months and years, I did find it helpful to know she had regrets. Now, I think an apology was a good thing for her to offer, bearing in mind she had betrayed our friendship.
Nearly 20 years on, she's still with her husband and I'm still with DH. We're happy, and I hope they are too. Our paths cross very rarely, but when they do we just say hello and move on.