Well, comparing between those two girls won't do anything good, at least that's what I think. Either way, you need to compromise about something, and at this point, everything seems toxic to me, so in the end, the ML has to act about the current situation so that the situation turns out to be better (well, typically, it won't for now, since typically there'll be many dramas unfolded).
And yea, I agree with her about putting him to his ex's shoes, he needs to realize his ex's feeling. I know that she did a bad thing and she also knows it. But in his ex's eyes, he chose her,who is more toxic than his ex, and just played along with her game. Ofc it would enrage his ex.
Last but not least, I also agree that it's FL who should be the one putting theirself in others' shoes. But at this point, it's already pointless in my POV. She is the one who controls all of this and has nothing to lose. She uses anything around her to make her in the top, she wavers others' feelings, makes many facades, etc. I think she won't have regret about doing those things since that is what she wanted. She can predict what others think and act yet no one can understand any act she does. She is one of the people that I call "natural winners of life", I know it's an exaggeration but after reading many stories having characters like her, I'm confident with it. Well, it's not that this is absolute, the real reason of her deeds can be unexpected and the whole situation can be changed, plot twist can also appear since it's typically there in this type of story.
That's just my opinion though, it's not that what I said just now is certainly true, so you don't have to really mind this comment, thanks.