
No necessarily. She planned this very carefully and she knew what she wanted and she had normal feelings--jealousy, desire, malice--and she went after what she wanted with full focus. At what point was she not acting with calculation, objective in mind, and careful to cover up so she wasn't seen as the bad guy. She's deceptive. She's jealous. She wants the guy she wants to be witih her. Those are not abnormal desires. The thing is, there are cultural norms and even moral ones that say: don't do that. She said fuck it to the norms and rules--like many people do in order to get what they want.
Greed. Selfishness. Jealousy. Deception. Those are moral and character issues, not necessarily mental illness ones.
If she had made up shit--imagined she was in an affair and loved when nothing happened, then we could say she was delusional.
But a person in a situation that makes them unhappy who connives to find a happier situation (for them, screw everyone else) is not necessarily mad. I"m very careful with tossing mental illness after every crime or rule-breaking behavior.
There's a reason we have norms, rules, taboos: People have inclinations to do selfish, crappy, violent things. And that's not madness. That's humanity.

It was implied. If a married man ignores the fact that he has a wife and agrees to meet you, date you and fuck you of course she will think he loves her more than his wife. Any woman would think he's not happy at home and I am special because he's willing to lose his family for me. This is a very common thing in real life.
All of this would have ended early if they just realized that Rina is a mentally ill person.