When someone's venting to you, their emotional state is somewhat compromised. They are seeking relief. They just want to let some words fly out of their mouth, and they want you to listen. You should take this as a compliment when someone vents to you because:
a) they see you as a deep enough friend they can trust you won't judge them for their (perhaps irrational) behavior
b) they know that you can handle them at a slightly ugly moment
c) they believe that they will find relief by talking to you
Rants can be about silly things like someone standing in a crowded doorway, the computer glitch that's giving you a twitch, or the fact that your neighbor just put a swing set in their front lawn. OR it could be something bigger like how your boss has been treating you like a nuisance or your mother-in-law invited herself over for 16th time this month. Whether it's big or small, we all need validation sometimes.
So you're talking to someone who's venting.
Rule #1 is DON'T TALK. Do you not completely understand what your friend is trying to express? Do you want to ask some clarifying questions? Don't!!! At least not in the heat of the venting session. If you must, ask one. But that's ALL! A person who is venting is upset, angry, or simply annoyed. They don't want to be interrupted, so if you interrupt, it. had. better. be. good. Interruptions only tend to frustrate the venter more, and if that happens, they might just shut up and bottle up their feelings. Trust me, you don't want that.
Rule #2, keep eye contact. This shows that you're listening and that you care about their feelings.
Rule #3, do not exacerbate the situation. Some people tend to reciprocate the mood of others. If you respond to someone who's venting by venting in return, it's a recipe for stress. Rather than talk loudly, quickly, and excitedly with the person, just listen. Calmly nodding in understanding can diffuse a tense situation.
Rule #4, wait. It might take a little while for your friend to get her feet back on the ground, but she will soon enough. Don't rush her. Rushing her might make her feel like you weren't listening in the first place or that you don't care about how she's feeling. She won't feel validated. So just wait. She'll get there, and if you just give her a little time, she'll feel so much better, and your friendship will be stronger.
Rant over. ;-) Thanks for reading.