Your job is not to give advice (although a little advice might be helpful.)
Your job is not to make them feel better (although your presence may comfort them.)
Your job is not to know the right thing to say (in fact, sometimes silence is the perfect response.)
Instead, your job is just to be with them. Give them a space to express their feelings. Give them a space to talk if they want. Give them the assurance that even though you can't fix their problem, they don't have to face it alone.
Henri Nouwen said it better than I ever could:
“When we honestly ask ourselves which person in our lives mean the most to us, we often find that it is those who, instead of giving advice, solutions, or cures, have chosen rather to share our pain and touch our wounds with a warm and tender hand.
The friend who can be silent with us in a moment of despair or confusion, who can stay with us in an hour of grief and bereavement, who can tolerate not knowing, not curing, not healing and face with us the reality of our powerlessness, that is a friend who cares.”
It's all right to offer advice now and then. But for the most part, you should say things like “I don't know how to fix it, but I'm here with you” or, “That sounds really hard and I'm sorry.”
If you're not sure what to say, try reflecting back to them what you're hearing. For instance, you might say something like, “It sounds like the stress just keeps piling up and it feels overwhelming” or, “it sounds like you're really frustrated with the way you're being treated at work.”
You can also ask simple open-ended questions, such as, “So what are you going to do now?” or, “How are you holding up under all this stress?”
Remember, you're not a counselor, and it's not your job to lead your friend to new insights. Don't ask leading questions or dig into their subconscious. Instead, just do your best to give them a safe space where they can process how they're feeling.
Next: It's not about you