Sorry can we stop absolving the OW please?
Timing absolutely terrible on her part.
And yes, maybe sometimes OW are told a pack of lies, and are guilty at most of being naive but in this case? Not buying it.
Any affairs between colleagues come with at least some idea of the others' homelife surely? She could have easily found out his actual situation from someone else (as she did eventually) and called it off early.
I expect she either got dumped (and was furious, causing max damage) or fed up of waiting and took matters into her own hands.
Either way she's an arsehole, as is he.