Not to be touchy, but someone trying to change how I dress is likely to set off a ton of red flags in my book. Not saying the guy isn’t a slob and couldn’t do better, but he is who he is.
“He is who he is” isn’t an excuse to be a slob any more than it’s an excuse to be a serial killer. If a man has a distinct style, and a woman comes along and tries to change it, that’s definitely worthy of a red flag, but dressing like a slob isn’t a style. It’s a lack of style.
If a grown man deliberately cultivates the look of a high-school stoner, then yes, that would technically be his style, but that’s not what’s going on here. (If it is, that’s a whole different set of issues.) What’s going on here is apathy. That’s what this woman means when she says her man dresses like a high-school stoner. The dude just doesn’t care.
That’s not acceptable to her, and the only thing she can do is try and get him to start caring about his appearance. Maybe he’ll start giving a shit. Maybe he’ll tell her to go fuck herself. Either way, she’ll learn something about the content of his character.