One important thing to remember when being a listener: it's very easy to make the mistake that you're supposed to solve the other person's problems. It might be that the other person isn't actually looking for advice, but rather just sympathy and a reassurance that there's someone who will listen to them. Try to figure out which one they are after, and remember that they may shift from one mode to another and not be fully aware of which one they want, themselves. (I wouldn't recommend explicitly asking them unless they are very, very Tell, for the above reasons as well as for some other reasons of which I have an intuitive hunch but am having difficulty formulating explicitly.)

Some caution is warranted even if they are looking for advice - typical mind fallacy means that it's easy to come up with a theory of what's wrong with them and how they should fix it that's vastly overconfident. If you suggest something that you think might help them and they disagree, then even if you were right, getting into an argument over it isn't the way you want to proceed.

Instead of trying to come up with a perfect solution, it can be better to just make lots of sympathetic comments and ask clarifying questions that are aimed both at giving you a better understanding of the problem, as well as helping them to think more clearly about it and resolve things where they've gotten stuck. Do offer suggestions, but offer most of them in the spirit of "here's an idea for you to evaluate and think about, that might or might not be useful".

I often figure that even if I can't say anything that would help directly, just helping them dissect the problem better and offering them new ways to think about their issue can be just as useful. Recently I had a phone call with my friend where we talked about my problems in general, and afterwards I knew exactly what it was that was the main cause of many of my problems, even though we never said anything about that cause during the conversation. But just talking about the problems on a general level was enough to get me thinking about them in the right way.