One of the hardest things about dealing with Not-So-Nice's is the fact that they are injured humans. And Nice Gals like to take care of the injured. We should have compassion for the injury, but refrain from enabling the NSN to continue his ridiculous fictional narrative of what is going on. That's not so easy.
It is important not to turn a NSN into an enemy, because by doing so we perpetuate the dynamic that is driving them to behave not-nice. Win/lose, etc., etc. ad nauseum. I feel the best way to accomplish this balance is to simply live by example. Live truthfully, and with integrity. Stop pretending awful things are ok, or normal, or justifiable. That is really hard! I know I had a lot invested in shoring up the pretend world/identity of my NSN's. But by causing him to face the real world consequences of behaving like an ass, you help them see reality...even if only for a few days. Their pretend world will never satisfy them - never! - and by perpetuating its myth we hurt both ourselves and the NSN.
At their core, NSN's are tremendously insecure. This is usually as a result of a psychic wound from their childhood. Because they lacked the coping mechanism to deal with the injury at the time, they create this false construct of the world to make themselves feel strong. Our Nice Gal is part of that engineering. Whenever their false construct is threatened (as it inevitably will be, it's false, after all) they lash out, usually at the Nice Gal because, hell, she's there and he has already convinced them both that she is responsible for his happiness.
At some point a Nice Gal may decide she's had enough. She tells the NSN to take a long walk off a short pier, dusts herself off, picks up the pieces of her shattered life and limps away. But if she looks back over her shoulder (I don't recommend this) she'll see a little boy. In that moment of truth he is taken back to when he was originally hurt. He stands there, defenseless as he was when the original injury occurred. It will gut our Nice Gal. It will trigger every urge she ever had to nurture and caretake and console and love. And enable.
Because I had a child with a NSN, I have the frequent opportunity to witness the recalcitrance, the sorrow, the pitiful abandon he feels. I also get to observe the serial job losses that are a result of his inability to manage his fear and emotions. I feel the pull to lend money, to soothe, to make excuses. To enable. To pretend what is happening is something different than it is. What is happening is a failure to grow. A failure to face reality. Oh, he will read an Internet article, or maybe even a book. But will he sit down and face himself? Will he say "this isn't and hasn't been working for me and I need to figure out what I am doing to screw up my life repeatedly and in the same fashion?" In this case, nope. It's always easier to blame someone else: a relative, an Ex, an employer or co-worker. It's always someone else's fault but the results are always the same.
But each failure is not just his punishment, it is his opportunity. This is the time when he could truly reflect on himself, to return to the injury, address it and heal himself. Or he may just move on, finding it preferable to continue the lies with another Nice Gal (there appears to be an inexhaustable supply, IBTP). At this moment he has a choice, and the likelihood he will do the work is directly proportionate to the degree to which she can stick to her guns. And so I do. Bang.