Validate their feelings: “I can see why you would be frustrated by …, I would be too!”
Be empathetic: “I am sorry that you are going through this!”
Support non-verbally by just sitting nearby.
Don’t:
Don’t minimize the situation: “This is not a big deal.”
Don’t minimize their emotions: “You are over-reacting, this is not worth getting upset about.”
Don’t correct their emotions: “You should be mad, not sad!”
Don’t say or imply that others have it worse: “This is nothing compared to …”
Don’t criticize or kick while down: “Why did you say …?”
Don’t say or imply to look at the bright side, to count their blessings, that it is going to be fine. It will probably be fine but this is not the point. The point is to help them process their hurtful experience.
Don’t try to cheer them up. This leaves the experience unprocessed and likely to continue causing problems for them.
Don’t act as the impartial judge or side with the enemy. Try to find at least one small way of being on their side. If you can not, do not comment and just listen quietly.
Don’t withdraw from the situation.
Don’t get defensive. It is not about you. Let slights go. Just listen.
Don’t switch the conversation over to another topic or to yourself. Keep the focus on their issues and feelings. It is OK for them to switch the subject.
Don’t ask leading questions that imply a solution: “Why didn’t you just walk away at that point?”
Don’t give advice or problem solve. If you have an idea, after the entire conversation is over you can say something like “I have an idea on how to fix …, let me know later if you want to talk about it”. Give them an easy out to not asking your advice.