Bruh, Ari a victim tbh. She was told by that lying ass Mincheol that Na Haesoo was just a crazy ex-wife. Not knowing she's actually being treated as a side-piece plaything, bound to be thrown away, leaving her with a brokenheart and love trauma later on.
Sad that just cuz she's young and naive, she apparently has to "learn" about the real world through this, which is basically saying you have to experience being used and abused by evil people just so that you could know how terrible the world can be.
Not saying everyone should just be naive, but those who aren't aware of all the bad things in the world don't deserve to be "taught" a lesson in reality like that.