This is a very difficult question to answer, it really depends upon the situation. If playing along can endanger the loved one or others then obviously you can't play along. However the bottom line is that you can not "talk someone out of" the delusion. The best you can do in a potentially dangerous situation is to re-direct the person to something else.
Carol Bradley-Bursack talks of her experience with her Dad who has dementia and lives in a nursing home. Her Dad became really upset one day that he hadn't received his medical degree from medical school. Her Dad had gone to medical school but never completed the program. This delusion lasted for days and he was angry- yelling at staff and other ;patients. Carol created and printed up a "medical degree" for her Dad and presented to him. He calmed down immediately and was fine after that.
I love how the article's author - Carol Bradley Bursack was able to play along with her Dad's dementia. It helped to calm him and make him as happy as possible in an otherwise very difficult situation. Carol talks about "validation therapy" and its use in patients w dementia. It can be a very effective tool to help the person w dementia become calm and untroubled again. #dementia #dementiacare