Negative co-workers

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I have a co-worker that I don't see very often because she works at a different office than I, but it seems I only hear from her when she wants to complain. I'm actually very good friends with her, but she's not easy to work with. When I worked in the same office as her - we'll just say she didn't make it hard for me to leave when I was offered another position elsewhere. How should I tell her that I've got more important things to do than listen to her complain about her boss and the people she works with (they come to me about her too). I have been able to remain neutral at this point - I have a "listen only" policy with them and I tend to make very few comments. Anyone have any suggestions? I do value our friendship and i don't want it to suffer because of this.