I think if I come at you with the express purpose of venting, then just some validation is really all I'm looking for, personally. A "that sounds hard" or "dang, dealing with X sounds rough". Things like that make me feel like you're actually listening and like I'm okay to feel so frazzled about whatever is happening.
Another approach that I think is good is asking whether or not whoever is talking to you wants validation, or solutions. I'm guilty of always offering solutions when people want to just vent and be heard, so that's always a good stop gap.
But I absolutely understand how awkward it can be trying to be there for someone and, of course, your results may vary. Still, communication gets us everywhere!