A question that commonly comes up when caring for someone with dementia is "Do I have to tell them the truth, even if it will upset them?" This especially arises when the person has forgotten that a loved one has passed away or some other painful event has occurred.
Caregivers quickly notice that telling the person with dementia over and over again that this sad thing has happened is very upsetting to both sides...it's hard to hear, and being the bearer of bad news is an unwelcome task. It can also be hard for you to have to revisit an event that might have hurt you as well. And it makes sense that it would be painful over and over again for the person with dementia: every time they are hearing it, it is as if for the first time. So it can feel cruel to keep breaking their heart with each telling. And I would agree that it is.
It can also be so hard to feel like you have to "lie" to a loved one, but when telling them the truth over and over again because they've forgotten hurts them, you don't want to do that either. I advocate for a middle ground.
In most cases, I would be sure they've heard the full truth once, because they have the right to know about the important things that happen in their lives, even if they are painful. It is part of being human, and they are allowed the fullness of that experience. In telling them the truth, you would want to be careful to take into consideration what they can take in and be sensitive to their cognitive abilities--simplify your language and the information, go slowly, repeat as necessary, and of course offer support and comfort.
But after that, if they ask again and appear to have forgotten what you've told them, I would use other approaches to spare them, and yourself, unnecessary pain.
An approach I like whenever possible rather than answering the question directly is talking about the feelings behind their question. If they are asking about someone who has died, simple statements like "You really miss her" or "It would be so nice if he could be here" or "Having her here would bring you a lot of comfort," can give them peace, especially as they lose their ability to put their feelings into words themselves.
You can also use it as a springboard to talk about memories they have of that person that they can still access. If they are at a point where pulling up memories on their own is hard, you can share some of your own and see if they can join you. "Do you remember the time...?" or "I always loved the way she..." or "The trip we all took to Disney was really special." It's also okay if the memories that they have don't seem entirely accurate, as their brain might be doing its best to fill in the blanks.
Or if they are being insistent that some task be done by someone who has passed away (or even just isn't available), being kind, reassuring, and gentle but firm can go a long way. Simple saying "I'm so sorry he can't be here to do this right now. I'm here. You're safe," validates that you hear them.
A very helpful add-on strategy to all of the above is redirection to get their attention elsewhere. Especially if they seem "stuck" on a question and ask it often. You start by meeting them where they are in the conversation, and then help them shift direction. For example, "We always loved going to the beach together. What do you like about going to the beach?" and then continuing the conversation about the beach...or whatever you think interests them or gets them engaged elsewhere. Or "You really miss her. I miss her too. You know who I've been wanting to see this week?" and continue about a friend who visits. You can also redirect them to an activity that will engage them.
These painful questions can be a tough issue. Finding this middle ground where you don't have to hurt them with the truth (more than once) and don't have to lie to them either can hopefully give you more peace of mind in your interactions with your loved one.