(*photo credit to Blondinrikard Froberg*)
Research, research, research: before you open your mouth. Before you offer advice. Before you say the thing, the wrong thing, because unfortunately, that would be the easiest thing in the world for you to do. Listen to them. Ask them questions, if they're comfortable talking about it.
If you're not willing to try, to educate yourself, to accept: to understand, somehow, then walk away. Let them be. They will wear your ignorance as a scar on the heart.
Please, please, please do not ever tell them to 'calm down', or 'chill out'; do not ever tell them that they're being silly or have nothing to be anxious about. Let's acknowledge the obvious: if they could simply switch it off, they would. They would choose, in a heartbeat, to live without the bastard if they could.