The key to helping a person have positive thoughts is to focus on the good in life. That might sound like an oxymoron but there is truth to this objective. It takes an effort to uncover. For instance, every day we're alive we see the sun, moon, stars, hear sounds and touch and feel. Things that make us glad to be alive. True, we might have difficulty with our limbs but we can still function. Wouldn't you say this is evidence of a Creator who loves us? Rather than being consumed by negative thoughts try to steer the conversation to the positive aspects of life. It will take effort and probably a few tries but you might want to start by asking them to think about the future. Ask: Do you think our world will ever get better? Will it stay the same or get worse? These questions hopefully will get a person's attention from himself or his current situation to something else. Once you've elicited responses you can direct them to a satisfying answer from a truthful, reliable source - the Bible.