Until Sickness Do Us Part
When you can, say "Yes, I will"
When you cannot, promise nil.
Broken promises are lies;
Liars all wise men despise.
Be thou in deed to every promise true,
Refuse to promise what thou canst not do
The question was "What is your understanding of the Buddhist position with regard to the recent preaching that it is ok to divorce your mate if they come down with Alzheimers Disease?"
There are several issues in this case.
The person seeking the devorce, having made the vow to remain in the bond "in sickness and in health" would be doing the equivalant of lying. If they have obtained in the marriage to this point the satisfactions they hoped to attain when taking the vows, this is also theft. This is bad kamma that can result in madness in this life and if made an habitual practice can bring one to hellish rebirths. There is no choice between living out a life with a mate that is becomming increasingly unpleasant to live with and telling a lie. Living out one's life in unpleasant circumstances will not bring one to hell. Telling lies can. A person finding themselves in such a position should accept the fact that they have some unpleasant kamma to live out and that there is no escaping kamma by telling lies for the sake of escaping the unpleasant.
The preacher is teaching the telling of lies which is the equivalant of preaching that there is no consequence to ones deeds at all because the lie destroys contact with reality and the liar having no basis in reality is capable of any bad deed. To teach such a doctrine and hang on to it tightly not letting it go will bring one to a hellish rebirth. To see this, imagine such a preacher facing after death those to whom he has taught this doctrine who are themselves suffering the painful consequences of acting on his instructions. Imagine what they wish for him.
As for the one being divorced, unless we know, we do not know and we can have no reasonable opinion. Such a one is experiencing some sort of bad kamma. Perhaps the outcome of lies they themselves told, vows they have broken.
As for us, to go beyond an inquiry into the ethics and form an opinion as to the goodness or badness of these individuals would be to create unnecessary confusion in our own minds. It would be better to remain a detached onlooker. Unless asked for one's understanding of the Buddhist position on such a matter we do not need to comment on it.
What would be the case where there were no 'in sickness and in health' clause? That would be an entirely different case. It would depend on the promise made, the intent, and the understanding.
AN 8.40 Where the consequence in this life is said to be being subjected to false accusation and slander.
Telling falsehoods — when indulged in, developed, and pursued — is something that leads to hell, leads to rebirth as a common animal, leads to the realm of the hungry shades. The slightest of all the results coming from telling falsehoods is that, when one becomes a human being, it leads to being falsely accused.
AN 8.40: Results, Bhk. Thanissaro trans.
NEW: AN 8.40: Landing One's Self in the Pay-up, Olds, trans.
 For the idea: AN 7.68: The Fire, Olds, trans.
"...anyone who feels no shame in telling a deliberate lie: There is no evil, I tell you, he will not do."
MN 61: Instructions to Rāhula at Mango Stone, Bhk. Thanissaro trans.