1) This isn't about you. Most likely he's infatuated with the boss, whether there is or isn't anything going on between them. And most likely he's depressed and that infatuation is acting like a drug.
2) This doesn't mean you're off the hook - if there are things you know you are responsible for in the relationship, fix them.
3) You said he had a ministroke - strokes can sometimes cause personality changes. Did they put him on any new medications after the stroke? Medications can also cause changes.
