Im beginning to wonder if XH took up with this OW in particular just to get back at the man that tried to kill him.
Seems more probable that there really was something - EA or physical or just heavy flirting - going on between them and that's why her H shot yours.
Let's face it - most normal guys, if they were shot by the husband of some woman they had NO interest in and had nothing going on with, would probably avoid that woman like the plague.