isn't the one and her and Reiichi are just friends in her present
Then why lie about not seeing Reiichi since their marriage ?
The only things I can think of are, Reiichi has confided in Risako about how bad his marriage is OR Reiichi made a pass at Risako. Either of those would lead Risako to want to avoid telling Kaoru about it without being cheating.
That scene where Kaoru's sick is written so coyly; it's annoying. I've seen a kabillion scenes like that, where someone talks about "these feelings for someone" and we think we know how the story's going but then whooaaaa it was Shiori's picture inside the locket all along! If Risako was straightforwardly just into Reiichi and that was it, it'd be very easy to have her state it in dialogue without all the ambiguity.
Thing is here, though... I can't tell how savvy the author thinks the readers are. Risako being in love with Kaoru has been strongly, strongly suggested by this point (and she's not wearing her glasses when she goes to visit her, either) but it's gotten to the point that the twist will be such a foregone conclusion, I'm concerned it'll be a double fake-out and she was never in love with Kaoru at all. (this would be stupid, and not just because less yuri)
This chapter does give us insight to Risako, though, especially that sick scene. Her strategy to help Kaoru sucks because she has no idea what is going on because she's like 16. "Hmm, I feel bad when she pays attention to Reiichi, but I feel great around her the rest of the time. I know, if I pull his attention away, there won't be a problem anymore!" She's flailing, but she looks so together, you'd never guess it from the outside.