Try to understand the customer's perspective. Maybe they've had other bad experiences and expect this one to be no different. Try to gain the customer's trust so they know you do understand theie perspective and you can help them see that this time, they won't go through the same negative outcome they did in the past that has given them the attitude to first had when they came to you. Smooth ruffled feathers and then build trust and confidence in the customer. How you do this depends on the specifics of this situation but when people are emotional they are also not very rational. Get the emotions under control and they'll see they can be reasonable and will most likely walk out feeling happier and smarter too.