I read a wonderful book called Contented Dementia a couple of years ago, and the author suggests (as others have done) to come up with a story that will reassure your mum and give her some explanation as to where he is that would put her mind at rest, that is in keeping with the sort of thing he would have done when he was alive and therefore believable. You might want to have a few options up your sleeve but they can be recycled as often as needed. The author goes on to suggest the very best solution is finding a story that would not just reassure your mum that he is safe and only temporarily absent, but one that would actually make her feel proud or happy for him and generate positive emotions. Examples might be that he is helping someone out with something specific he used to do regularly or doing something he was very good at and used to enjoy. I've not had to use these techniques myself yet, but I read these suggestions with interest as it was revolutionary to me and also the idea of lying didn't sit well with me at all (it horrified me to be honest!) until I was really able to understand the ramifications and put the other person's needs above my own. The author spent a lot of pages stressing the importance of taking every opportunity to generate positive emotions, and avoiding causing distress, regardless of the reality, because the brain of the afflicted person just isn't capable of processing reality anymore. Therefore any information that causes them distress is pointless regardless of whether it is true or not, and extremely detrimental. The main premise of this book is how to give someone with dementia the best quality of life possible regardless of how advanced their symptoms, and that being detached from reality does not have to mean a life of misery. Doing or saying anything that makes them feel good and result in positive emotions will improve their quality of life dramatically, and if that means setting aside our own deep seated ideas of what is is right and wrong (i.e. we shouldn't lie and should be open and honest) then so be it, as the discomfort we might feel should remain our problem and for us to deal with, not their problem. Being kind and compassionate trumps being truthful in this situation. I thoroughly recommend that book, I found it incredibly inspiring and reassuring and it is full of practical suggestions. The author is Oliver James and he's an absolute hero in my mind.