Say things like: "you're completely crazy" or "this is true madness", or "you need a shrink". Don't try obsessively to change his/her mind (you will only make him/her feel more anxious).
On the other hand don't say things like: "you're right", or "I can see it" if you don't think so, or things like: "Me too", just to make him/her happy. Also, don't make a joke of his/her perceptions and sensations; do not minimize them. Do not laugh, nor be aggressive or angry.
Listen attentively giving him/her all your attention, then reply that it's really difficult for you to understand such a deep inner experience. Try to show, cautiously, the difference between what s/he pretends to see, hear, feel and the objective reality that one can touch and feel.
Be empathic and patient and honestly try to see the things the way s/he does. Then try to move conversation towards another thing or try to start an activity that requires concentration and attention in a different field (like asking him/her for help you to do some housework, some gardening work, some cooking work.
Suggest to call Dr. XX (psychologist or psychiatrist, or social worker) and to have a word with him/her about these feelings to ask his/her opinion about them.
And last, but not least, do not panic, every problem has a solution!