Are there permanent ill effects from taking a person with dementia on a trip?

My son is getting married in NY later this summer. He very much would like to have his grandmother, who lives in TN, attend. Although she has mild dementia, she is in good health, and wants to attend the wedding. She would be driven to NY by a family member, as she is too mixed up to fly by herself. However, her caregivers say that every time Grandma takes a trip or vacation, she returns to the retirement home in a more mixed-up state, and doesn't ever regain the ground she has lost. I see that she is declining somewhat, but isn't that the nature of the disease? Is any decline caused or accelerated by the traveling? If there is any decline from traveling, is it permanent, or can she regain that loss? Are there occasions worth the possible decline so the patient can attend a function? If so, who makes the decision whether she can travel or not? And finally, when do you stop taking what the person wants to do into consideration?