It isn't your job to know what to say. It isn't your job to fix a situation, and chances are, your words wouldn't have fixed anything anyways. Please stop kicking yourself because you can't find the right words, or are scared of "saying the wrong thing". In most situations, there isn't a wrong answer. You are trying your hardest to be there for them, and they know it. You're gonna mess up, stumble over words, and misinterpret things. That's okay. You're doing your best with the circumstances given, and that's more than enough.

People just want to be heard.

You know that cliche saying "you have two ears and one mouth for a reason"? Yeah, well, it's so true. People just want to know that they have someone to talk to. Be that person! There's nothing wrong with it. But when you listen, make sure you're listening to understand, not listening to respond. Focus on that person and what they're saying. Be engaged. Show them that you care. Sometimes, an ear to listen or a shoulder to cry on is more than enough. Unless you are physically God, everyone knows you're not going to have all the answers or solutions, so don't try to. Listen, and be there for those you care about.