In a nutshell, I was the OW, OM's wife found out, told me I should tell my H (which I did) but turned up at my house to tell him anyway, as she didn't believe I would. H was out at the time (I'd told him and he'd gone out for a drive).
Quite frankly, she made it appear that everything the OM had previously told me about her being a crazy vindictive nutjob was true. I'll never forget her standing at my front door, looking and sounding completely unhinged.
My H had no interest in speaking or communicating with her, he ignored her calls and texts at first but she wouldn't let it drop. She didn't do any of this out of a sense of fairness, justice, wanting the best for my H, feeling he should know. She did it to hurt me, to spite me, to try and break up my marriage like I'd broken up hers and in the process she caused my H extreme distress, I thought he may have some kind of breakdown at one point due to her harrasment.
6 months on and H is directing a lot of his anger towards her, and yes I realise this is completely misplaced. He has even recently (jokingly, I think) commented that he's not surprised that OM was having an affair, being married to a psycho like her.