It's tough to talk about Alzheimer's Disease when your loved one suffering from it is right there with you. I'll have to admit, we didn't discuss it much with my mom. In hindsight, that's a mistake. Any person suffering from any illness should be a part of any conversation you have about it.
For example, yesterday I visited my friends Don and Carol. While not definitively diagnosed yet, Don is suffering with some kind of dementia -- early stages. There is no whispering behind his back or talking when he's not in the room. He'll discuss it, as will Carol. Likewise for my friend Mary Ann. When her husband was diagnosed, there was no talk behind his back.
Be honest with your loved one suffering from Alzheimer's or dementia. Your support will help them accept the disease, and do things to help slow it down. They do know what's going on -- not talking about it may lead them to believed you're ashamed of them. Post diagnosis, the road is long and not easy. Be there for your loved one and reach out to other family and friends to be there for you.