The manga didn't mention any thing about cases where domestic abuse is involved.
If cheating is sufficient enough to secure favourable terms for divorce then there's no reason to think something as harmful as domestic abuse wouldn't be a sure guarantee
Would depend on what Japanese Law stipulates as 'domestic abuse' and whether it can be sufficiently proven to the court as clearing the bar to necessitate dissolving the marriage sans consent from Itsuki (in Rio's specific case).
Without knowing that, it becomes difficult to know Rio's chances of actually succeeding in removing herself from her marriage.
That's not to excuse what Rio did. But it's not even legally clear what recourse she had, realistically, to solve her core problem.
And there's the reality of emotional abuse and the feeling of futility. I don't know how this mini-series goes, but maybe Rio
did try to find a way out, but couldn't do so in a manner that didn't amount to simply running away and giving up everything in her life.
That might have proven too high a cost for her, and so she remained trapped with no other avenue open to her, and the constant belittling of her personhood by Itsuki wore her down and made it difficult to talk about her burdens with others further compounding her feeling of isolation, and e eventuallyher building resentment toward the other three, and especially the (seemingly) happy marriage of Mahoro.