Over the years Linda and I have been privileged to enter into marriages that are reeling from the shock of an adulterous, extramarital affair. When we are approached by a spouse who confides in us "My husband/wife is seeing another person," we are saddened and yet confident that, if this couple wants to, their marriage can be restored.
Their marriage needs to be and can be be better than it ever was. Such moral earthquakes always take place along a pre-existing fault line.
Every marriage - mine as well as yours - is either more or less affair-prone. When a marital couple stops investing in the marital relationship, intimacy diminishes. This is dangerous, since diminished marital intimacy (shared privacy; sharing spiritual and emotional life together) is a predictor of adulterous behavior.
When there has been adulterous behavior we recommend a book by Dave Carder - Torn Asunder: Recovering From an Extramarital Affair.