Honestly I feel like all three of them are to blame for this shitty situation.
Nanase not caring enough about Yuni (more like not giving enough attention) , Yuni turning to Fuuko for attention and care whilst being in denial she isn't doing a bad thing and it's not her fault (Which like...what? seriously bitch?) and Fuuko for initially leading Yuni to play along with her antics which, combined with Yuni's denial, led to said shitty situation. Though it's obvious that Nanase is the biggest victim here. and Fuuko maybe being somewhat a bit of a victim too, although very little.
They might be able to somewhat calmly defuse the situation if Yuni explains to Nanase her reasoning for cheating on Nanase (which is just being lonely) and after a hopefully somewhat civil argument they will reach a compromise / solution. although it won't make any of Yuni and Fuuko's actions justifiable.
Honestly this is my first (and probably last) NTR Yuri manga and I hate but somewhat love it at the same time.