I did talk to the OW, but just over messaging and on the phone. OW was my first and primary source of information about xWH's many many affairs.
What I think I did right: I asked her for verifiable facts - dates, times, websites, pictures. Everything she told me that I could verify checked out, which was mainly dates and geographical locations where xWH took 'solo' bike trips. She gave me the dates and places she saw him in person, the dates he told her he was getting STI tested, she gave me his username on the website they met on, she sent a screengrab of a text convo where they were going back and forth working out dates to meet up at an AirBnB, and sent a screen grab of one of their video calls with xWH clearly pictured laying in our bed and her scribbled out face in the small window.
Later, I called her using the sketchy Telegram app she and xWH had used for affairing (you don't have to give the other person your number, they can call you or video you using a user name) and I tried to trip her up a bit by repeating a few things back to her incorrectly, but she corrected me with the info she'd given the first time.
What I wish I hadn't done (but maybe was for the best): I asked her what xWH had said about our marriage. And what she told me did track with some of his posts on the cheaters chat board (good marriage, he just 'wasn't monogamous' in his heart), but I also discovered he'd told her all about me. Where I grew up, went to grad school, my career, siblings, things like that.
Back to something I think I did right: I asked her to send me copies of their texts in which he talked about me/our marriage and she took a few screen grabs of their texts while we were still on the phone (so she had no time to edit, photoshop, etc) and sent them to me.
I don't know nor care what her motives were - if she wanted to get back at xWH for something, if she wanted to hurt me, if she was a drama queen trying to insert herself into the chaos of affair discovery - I didn't care, because all I wanted from her were facts. I've reached a point in healing where I am nearly grateful to her for being SO damn creepy that she downloaded and saved two years of affair texts and pictures and regularly took screen grabs during their video sex. Maybe she wanted to financially blackmail him, he said no and she pulled the trigger on telling me, I will never know. She told me she was sorry, but she didn't sound sorry, she sounded like she wanted to keep me on the phone and dish some more about the affair. That's when I hung up and have never contacted her again.
On balance I'm glad I got the information and I don't regret talking to her, but my xWH was the kind who stonewalled and did not give me any information I didn't already have proof of (and denied a lot of things I did have proof of), so everything she told me was, for a while, everything I knew of his affairs.
If you're in R, I'm wondering if you want closure in the sense that you want to hear her motivations, or if you want to know if you do know everything, if your husband was totally honest?
Because I think talking to the OW is great for fact-finding, as long as you are asking for facts you can verify, check, or ask her for proof of. But I think it's a bad idea if you want to understand the affair by hearing her side of it. I don't think there's anything they can say that will make you feel like 'oh, that makes sense, I see how that happened now' and you might instead hear things you can never unhear.