This is a very common experience. Sometimes early in their dementia people will have an occasional moment when they forget someone they know is gone, and you can gently nudge that memory back. As dementia progresses, they might start to think more often that they are living in a time in the past, when they were younger and those people were still with them. At that point, reminding them of that reality can go badly, triggering grief or even anger at you. At that point, if your loved one casually mentions to you that they had lunch with an old friend who is no longer alive, let the statement go. If they are distressed and looking for their mother, consider what underlying need might be driving that. Needing your mom is a very relatable experience when you don’t feel well, are scared, or lonesome. Ask if something is troubling them, and how you can help.