What kinds of things have you said to partners or would-be partners who treated your partner with a lack of respect, insulted them, or engaged in trashtalk? Do you automatically break it off with such people, or do they get a warning?
I have a zero tolerance rule on this, and so does my H. Many years ago he tolerated someone trashing me, being conflict avoidant just as you say. He hated it, but didn't set firm boundaries about what kinds of communication were and weren't acceptable. He realized much too late what he'd done/enabled.
By the time it was necessary for him to break that off, the other party thought she had established a standing with him (i.e., discarding/trashing me as a communications pattern indicating intimacy/relationship privilege/status) that was not the case. He was shocked to learn this, and shocked to realize that his failure to set firm boundaries had contributed so strongly. I was even more shocked. It was a painful, devastating lesson for both of us, and especially him. I went through a period of fury that I had to be the one to set those boundaries and hold his feet to the fire. I found just how strong my sense of justice was: that above all we must be loyal to those we love. That was the real hurt for me, that he didn't stand up for me, that he wimped out.
The example I used with him, in the fury of our working this through and out, was based on his military experience with top secret security clearance. (He often processes things like an engineer...being an engineer.) I pointed out that if he had blabbed in his military service the way he blabbed in his relationship, or gave away privileged information, he would have been court martialed. Or terminated.
That woke him up. He was shocked to see what a violation of loyalty he'd allowed and contributed to. He could see that, although he was not trash-talking me with her, she had strategically and tactically deployed certain facts (such as that he and I were working through a dense period of conflict over decision making) and built a gradual and escalating pattern of disrespect on that. When he confronted her on that, she showed her hand, and he ended it 100%.
My point there is that the "trash talking" isn't always clear at first. The disrespect can start in more subtle ways. For instance the criticism of the non-present party might not be calling her a bitch or him an asshole, but putting them down more subtly.
These are ploys for relationship power and leverage, and if they are not nipped in the bud, eventually they will get ugly. So zero tolerance is, IMO, the way to go, and by the time the real trash talking has begun, there is probably a long established tradition of other disrespect, shaming, jockeying for power, bullying, etc.
There can be absolutely no tolerance in intimate relationships for vicious speech; it is a form of violence. Anyone who does that against a third party is asking others to conspire in it. They must be brought up firmly and quickly, and if they cannot control their feelings and speech, I want nothing to do with them.