[Last] week on his television show Christian broadcaster Pat Robertson said a man would be morally justified to divorce his wife with Alzheimer’s disease in order to marry another woman. The dementia-riddled wife is, Robertson said, “not there” anymore. This is more than an embarrassment. This is more than cruelty. This is a repudiation of the gospel of Jesus Christ.
Marriage, the Scripture tells us, is an icon of something deeper, more ancient, more mysterious. The marriage union is a sign, the Apostle Paul announces, of the mystery of Christ and his church (Eph. 5). The husband, then, is to love his wife “as Christ loved the church” (Eph. 5:25).
At the arrest of Christ, his Bride, the church, forgot who she was, and denied who he was. He didn’t divorce her. He didn’t leave.
There is probably some support for someone whose spouse is, "not really there" seeking other companionship. Certainly there is sympathy, and that sympathy itself is not wrong. And just as in other areas of sexuality, there is the selfish view that one's personal 'needs' outweigh what is right or wrong. It's as if we are justified in including a, "but" after every vow.
Is a person justified in leaving a spouse who has Alzheimer's? I guess it depends on how seriously you take your promises.