Love how everyone absolves this other woman. Sure, she didn't make marriage vows to Roxylou, however, she did embark on a relationship with her DH, knowing full well that he was married with kids. Of course it's going to be jarring, when alongside this, she is portraying herself to the world as a woman who supports other women, bla bla bla. It's hypocritical and totally shitty, and I would feel the same as Op.
I'm not sure there's much you can realistically do though. What did you have in mind? My best friend shagged my ExH (while we were together). Quite honestly, I had day dreams of stabbing her tyres or throwing a brick through her window, I was so angry. Of course, the only loser there would be me, as I'd get caught and possibly even arrested, so the only thing I could do was to never talk to her again. Years later she got very serious with a guy who let her down monumentally and she lost her home. I'd be lying if I didn't have a smirk.
This OW will be in deep shit with her husband. And I bet she feels so mortified that she risked her own marriage for a man that ultimately picked his wife. I bet she feels like a right saddo.
When this happened to me, Facebook and social media was only in it's infancy. If it happened now, yeah, I'd probably name and shame her on-line. But I'd not do anything more troublesome. You have to be careful as well, not to implicate your own DH in any posts, or you might be telling the world what he did, not just shaming her.