She finds him smoking on her porch, she hates the smell. Last time she saw him doing it he was with Faith, she should have said something then but there was the dead girl and a smirking Faith and it would have sounded like an excuse for misplaced jealousy even if nothing relating to Faith and Spike and cigarettes would ever pass as innocent.Smoking won't kill him anyway.
Now she wants to bum a cigarette as a sign of peace, 'cause he's not smiling and looking at him she feels that the smell of nicotine on her skin would never be as bad on her as Eau de Angel is. She sits by him, he breaths out smoke and says 'we all have our filthy habits, love'. And she knows she's forgiven, or at least put on hold, 'cause the world is ending and he's out of fight and like the bad habit she is, he'll always come back for more.