Risako's probably vaguely in love with Kaoru, but more importantly, Risako can't stand bullshit and beating around the bush (which is Kaoru's main personality trait). She clearly does not think Reiichi is worth all this time and attention, and she's willing to be the villain if that's what it takes.
I think it is verrrrrrry unlikely Reiichi is cheating with her, not least because she was wearing her glasses when she was with him, and her glasses are a symbol of her being like "don't pay attention to my pretty face, assholes." (as an adult, she takes them off when around Kaoru. And when Reiichi is a huge dick and says Kaoru's "not very bright," that's what incites Risako to take her glasses off and seduce him away.)
So the only question is, is she playing the martyr and deliberately making Kaoru think Reiichi's cheating with her to try the same gambit she tried in high school and finally get her over him? Or is she just one of Reiichi's few friends and so knows he confides in her about his crap marriage?
Honestly, Kaoru/Risako makes way more sense to me than Kaoru/Uta. There's a random stray lesbian hanging around Uta's friend group, right?