No, an extramarital affair cannot make a non-narcissist a narcissist. Narcissism is a personality trait and a part of someone's personality structure. It is not something that can be acquired through external events like having an affair.
Narcissism is characterized by a grandiose sense of self-importance, a need for excessive admiration, a lack of empathy for others, and a sense of entitlement. These traits are typically stable over time and are deeply ingrained in an individual's personality.
Extramarital affairs, while they can be hurtful and have consequences for relationships, do not cause someone to become a narcissist. An affair may be a symptom of underlying issues within a relationship or individual, but it does not fundamentally change a person's personality structure.
It is essential to differentiate between someone who may have made a mistake by having an affair and someone who genuinely exhibits narcissistic personality traits. If you or someone you know is dealing with relationship issues or concerns about personality traits, seeking professional help from a therapist or counselor can be beneficial. They can provide support, guidance, and help navigate the complexities of relationships and personal growth.