I, too, think the expert missed the mark on this one. Perhaps offering pieces of an apple, carrot or celery at the end of each meal will help cleanse the teeth naturally will help when all else fails.
After each meal, have the person follow you to the bathroom to wash your hands and brush your teeth together. Mili has a good idea - demonstrating the teeth-brushing ritual after each meal, something the care-giver does anyway, could instill in the care-receiver a sense of belonging to the activity and could even become something "fun" (who can make the most bubbles) for both participants instead of just one more thing you "have" to do.
I totally agree with LindaSD in that reminding someone with dementia that they need to do something just reinforces the feeling that they are forgetting just one more thing. Try getting up after a meal and saying, "Let's go brush our teeth" instead of "Do you want to brush your teeth" or "You need to brush your teeth" might meet with more agreeable responses. Choices and admonishments just don't work with dementia sufferers.