Like he said, he hasn’t meaningfully progressed in the last X yrs. Being in a real romantic relationship with him would hold them both back. She needs to be with someone more stable and he needs do stuff for himself.
Actually, if she wants to accept the burden, she could save him. If he's just left alone, chances are majorly on the side of the dude not achieving anything and falling into deeper depression. But if she could keep him occupied and motivated to do something sensible with his life, focusing on concrete things instead of dwelling in the past, he could get over it. He would have immediately solid content in his life if they became a family. He could work hard for it.
What would she gain, though? She really should be able to find a better man with her beauty if she tried. But then again, it's not always so easy, plus she has been disappointed before. Maybe she saw him as a safe choice without ulterior motives. He's a pretty decent person, it's only his circumstances and prospects that are anything but good.