I also confronted OW, and told her exactly what I thought of her, when she was having a cozy dinner with H, alone, at my daughter's apartment. She tried to leave, saying "I don't deserve this", but I chased her down a corridor as she tried to escape, saying "Don't deserve it? Ha! I haven't even started yet"
When H tried to intervene, I told him I'd speak to him when he grew up. And I told him later to pack his bags and leave-
It felt good at the time, but didn't help. OW used some of my comments as a way of flattering herself and H (I'd said she was too awful to have friends her own age). She told H that men her own age (26) were just so immature, so she felt so much happier in his company. She also said that she didn't trust me at all, as I was capable of anything.
H said he thought I was always so unreasonably aggressive to OW. Yeah. I should invite her to be a guest speaker at a dinner for LBS in Portugal, and see what happens...
In short, it was temporarily cathartic, but in the long term, didn't help me, H or our M.
Newbies... another one to avoid.