"That's sounds horrible! I'm so sorry you're going through this." After they have sufficiently vented, ask if there's anything at all you can do to help. Offer a few small comforts (a seat and a glass of water'll do). Be physically comforting in the appropriate way for your relationship. Maybe just a soft hand on their shoulder. Talk calmly and soothingly to bring the general temperature of the conversation down. Then, just pay attention and BE THERE with them. This is what I always want when I am venting, and it seems to work for people I care about.