I don't think that it is at all confusing that there's a difference between these two situations:
a. Woman has no clue that she is OW. When she finds out, she is horrified and breaks up with him. Might phone wife to warn her and apologise.
b. Woman knows full well that she is OW and might phone wife for malicious reasons or to make sure the couple break up.
Obviously, we can't say "the OW should always / never phone the wife" as those two situations are so different. It's not that you can "never get it right". It's that those are two totally different things.
Of course, there are other situations, too.
c. Woman falls for web of lies man spins about how he and his wife still live together but it is all sham and the evil nasty wife actually even said that she would be OK with him sleeping with other women. Plus he is really sad about nasty wife's treatment and needs a cuddle. Woman might phone wife to tell her off for being so nasty to lovely man.
Personally I can't get too overexcited about my ex's OW as I know he was out looking for OWs before he got together with her. Sure, she has no morals / is a bit stupid, but as I don't know her I am indeed a lot more angry with my ex, who treated me badly in person on a daily basis for months. Kind of drowns out any interest in her morals.