I will like to throw in a theory if you don’t mind. The fact that none of the two individuals chose to put in an effort shows that they had already set apart even before the incident. If the relationship had been the priority, there would be efforts. The absence of effort by either of them may look like the soul cause, but I think it was just a symptom. From my experience (which is not enough considering I’m talking to a doctor here), a separation like this is never caused by one single event. On the contrary, the said event may bring out the compatibility and trust issues that had been there for years.