Listen when they talk. Phrases that acknowledge and validate like "this must be difficult" and "I can hear the (insert emotion they are expressing such as fear,anger or frustration) in your words"
I also think it's OK to tell people you don't know what to say. I also think it's OK to ask them if there is anything you can do or say that would help them. They may need someone to just listen. They may be tired of processing difficulties and need a distraction from it all.