Fvck him and the horse he rode in on. You're too good for him and don't you ever forget that. She's a skank and he's a skank-doer-coward. pffft.
Time to stop OCD thinking on this turd and go total zero contact (which means no looking on any of his social media sites as well) and do the mental work you need to do to consciously change the subject when he pops into your head. Start the process to get you to the stage of indifference to him.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion