I think it is initially natural to feel some angry emotions towards the WS' AP as a coping strategy. In this situation, I feel like our normal loyalties to our SO get mixed up and we tend to want to shift and direct the blame to the AP. Much better to direct those strong emotions towards health, healing, and reconciliation.
What helped me was to shift from a mental position of murderous anger towards the AP to a position contempt and dismissal. In most cases, these APs are sad and miserable people (that was our case). They are latching on to our spouses because they are incapable of creating and sustaining a real relationship. The only thing they can offer are their genitals and fake compliments. Once I started viewing my WS' AP as a bad sex toy then I was able to let it go and not waste mental energy on him. Because at the end of the day, it is really about what my wife does (her ability to cope with life, manage stress, and participate in healthy hobbies and relationships) that matters for our reconciliation to work.