I don't agree that affairs are acts of anger - mostly they are acts of opportunity and selfishness. It feels good so why should they care how it affects anyone else?
In her situation I believe it is all 3.
My wife very likely had symptoms of a wallowing or "covert" narcissist. Not the classic, highly manipulative, bombastic over the top, controlling type.
It's likely something that has been completely unleashed in this midlife scenario.
Everyone has a dark side, some people aren't at all able to keep theirs at bay.
