You are unlikely to be able to make them believe that what they are experiencing isn't real. As it turned out, I was experiencing a psychotic period due to my depression, which meant I was mis-diagnosed. I am now getting help for my depression, but I do still have experience of such things, but with the, well, benefit I guess of no longer feeling that way (hopefully never to be that way again). Looking back on it, what I believed was utterly ridiculous, but I totally believed it at the time, and couldn't accept that it wasn't true. What you are best to do is to try and accept that they believe these things, but just try and be there for them. Find things they enjoy doing, and just try to give them a good time, rather than spending all your time trying to convince them of something they probably won't accept. I don't know if I've worded it quite right, but I hope you understand what I'm getting at. Accept that they feel that way, and just find things that they can do and will enjoy doing. Just having someone there is a great help in itself, to anyone with any mental disorder.