The real problem starts when the venting is about the negative attributes about the person she is venting too... a real double bind.
my wife (recently diagnosed with bpd) keeps venting about me to me and the reason is "whom else can I vent to". It is extremely difficult to do a "hmm" or keep quiet till she calms down. extremely difficult situation I am in.
So I would really echo what Ducks is saying ... and I would add this little caveat. At one point in my relationship, my T and I discussed approaching conflict in this way. What I failed to comprehend (an this is my story - not every story is the same with the same dynamics) was that in the case of my partner, the effects of this were temporary until she wised up, found new things to move in on me, and the process started over.
Ducks is right - no one, for any reason, should stay in an abusive relationships. If you find yourself making any excuse for rude, unkind, demeaning behavior - that's a red flag that your boundaries a slipping.
When this happens, when you get caught in one of those "discussions" that have that sense of "venting to you about you" - what's the dominant core emotion that you are in touch with - happiness, anger, fear, disgust, surprise or sadness (or a combination) Underpinning this - what combination of shame and guilt are you left with. This too are ways to discern if it's time to take a step back.
Hope this helps everyone here. Thanks for the exchange. While I can't repair what is no longer there, this board helps me to heal.
Rev