Automatic Excommunication in 1967?

My father dodged the Vietnam War and we later found his whereabouts to be in New Zealand. My mother tried to get an annulment but was denied. She divorced for abandonment and said she was excommunicated. She must have told me this 50 times as it hurt her tremendously. She "secretly" attended and remained personally devout. Her second marriage was to a Protestant so I don't know if that played a part. My sister-in-law doesn't believe what my mom told me and thinks you can only get excommunicated if you committed a horrendous crime in a sane mind. Back in 1967, I think the Church was more cut & dry / strict, when it came to divorce. I know my mom had 'shopped' many other religions to appease her spiritually and for the need of community, to no avail. She was Catholic and there was nothing else! Please clarify this for me.