Seems like a lot of dudes get in situations like this by getting serious with girls who have mixed feelings about them. I'm sure there have been signs leading up to this that this girl didn't truly care for the dude and he cared more for her than vice versa, but the mixed signals of her sometimes being into him built false hope. It's just like the saying, it's not the people who like you or dislike you who cause you the most trouble. It's the people who have mixed feelings about you.
Most people would rather hold on to hope of the other person seeing their ways, growing empathy, and becoming "better" in due time than to be alone.
They learn the hard way that change, certainly as an adult, is hard. Change requires a human being to accept all the pain and suffering they caused others, and all the opportunities they've missed or messed up their entire lives being the way they were and having the worldviews they had.
If this girl is the sort of person who considers caring about others, even her own company when going out, as being inconvenient to her "doing what she wants" and "just feeling good" and "having a good time", than imagine the devastating existential crisis she would go through if she actually came to a genuine realization that she has lived her entire life being a self-centered person. That everyone she ever knew who told her that she is the toxic person and the hurtful narcissist here was right, and she was the one that was wrong. You would feel like you're entire life was a lie and "I don't even know who I am." The biggest sign of depression.
Most people don't even have the strength to admit their wrong about something they thought they knew while in casual conversation, even when the belief pertains to something inconsequential and impersonal (just walk into any coli thread). So imagine the emotional weight of realizing you've spent your whole life being straight up "wrong," a "bad" person, a "bytch". It's hard and it takes intense therapy. Most people are cowards and do not have the strength to reflect on themselves, admit to their character flaws, take in the guilt of someone they did wrong, and then work to do better. Certainly not some shallow attention starved thot. It can leave you questioning your whole existence.
Most people would rather hide and bury difficult feelings that make them uncomfortable, depressed, and questioning "who they are" such as guilt or self doubt or shame, than take them on and be better. There's hard ways through life that require intellect, learning, character, growth, and then theres the easy ways through life which is all about serving your own interest through thoughtless blind actions that feel immediately convenient and beneficial for you with no grounded forethought or conscious. Just taking life by each moment of convenience. and most people are bum asses that take the easy route and then wonder why they are miserable and always having issues.