It's called "social death."
In olden times, social death came from banishment and shunning.
In modern times, it's not so uncommon because our outstanding medical facilities allow people to stay "alive" for quite some time as mental acuity fades. Or, they get put on painkillers days or weeks or months before death, to ease suffering.
In either case, once friends and family start to sense the person is no longer "there" because of lack of coherence or lack of memory, they think of the person in the past tense.