We've all experienced talking to difficult people. At times you may want to slap them in the face but diplomacy is all about disagreeing with others and getting them to like it. It sounds like magic but here are some simple steps you can take to argue with people and get them to say "thank you" afterwards.
Step 1. Let them talk as much as they want. Anything you do or say can and will be used against you. So long as they're mad, frustrated, or angry it's a one-way conversation with them talking and not listening to anything you have to say. Instead, take the hits and endure the pain but never accept abusive language. Stand firm and let them know that as much as you want to help them, abusive language isn't going to help.