Yes, at no point have I said he should not be held most responsible. He's scum. I He'd be a predator, whatever her age, he chases other women and lies about it to his wife who trusted and believed him.
Her brain development isn't pertinent to this situation. If it was a quick, impulse choice, sure maybe, but she planned and executed her choices all along the way. This is a matter of character. At no age would she not have known that pursuing a fuckfest with a married man was unethical. At 21 she certainly knew it was ethically wrong, but made the choice anyway. And then went after the wife when she was spurned. She sounds like a nasty little cow tbh.
I said earlier maybe I'd have gone for it if the married man had pursued me at work when I was that age (I was actually thinking of one particular married man, an accountant at a company I worked for back then, who did use to flirt with me). But, no. I was thinking about it, and I pulled back from his flirting as that's just not in my character. It wasn't that I didn't want to, I just couldn't. It's not my programming.
I have come to believe, from decades of experience, that our character is pretty much immovable. There are some things we will not or cannot do unless our life depends on it. And we are all different in what our boundary lines are. And yes, people really do show you exactly who you are, and you should believe them.
She decided she wanted to make lots of choices that led to her choosing to have an affair with a married man, as an adult. She's responsible for herself. I hope she's very unhappy and learns a valuable lesson or two, but I doubt it, cynic that I am. She'll probably just keep making vagina based and "I want it and I will get it no matter what" choices and decisions, since that's in her character.