This all sounds really tricky - and really upsetting. Sounds like neighbour A is either uncomfortable about what neighbour B said - or is
tit stirring.
It seems to me to be such a common experience though. People often make up things to
witch about. If they're bitching about you, it's almost certain that they perceive you to be superior to them in some way. Small consolation but it gives you the upper hand!
You could ignore it, don't say anything more about it, sail on past whilst you heal your shins in private. You have the upper hand here and the choices, B doesn't - she's caught up in her petty inadequacies. The coffee meets would probably wind down because the previous warmth and trust has been eroded anyway. I tend to take that tack - like, do I really value that person's opinions/approval that much? Nah!
I've confronted very gently on some occasions - like at work where it's necessary to maintain a functional, productive team or dept etc. I once had a team manager bitching about me about something I was supposed to have said. I was her boss's boss. I had a quiet chat with TM, turned out to be made up, baseless gossip, storm in a teacup. We said no more about it. The interesting thing was that several few months later, I was called in to look at the spending and outcomes of that TM's dept. 360 investigation. Turned out that this TM had frittered away £several hundredK on a failing project. I shut the project down and she was moved - but not before she'd had her line manager up on an empty grievance complaint. What I realised was that the woman was throwing out these bitchy accusations as some sort of cry for help or cover for her failings. A big lesson for me: I should have dug a bit deeper to find out what was behind her initial bitching.
You probably don't want to take responsibility for listening to neighbour B about what's behind her bitching, nor for supporting her. I don't think I would! What I might do is diplomatically have something else to do on those Friday afternoons - as
@Chita suggested. A 10 week course of physio, or take up something like Tai Chi, a language course online, an art course at a local college etc etc. That couple of months away from both neighbours might save years of neighbourhood unpleasantness down the line. And anyway, you're looking for a way to bring these afternoons to a halt!
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I'm so sorry to learn what your partner's experienced. It's horrifying on many levels. It's such a shock to the system to be assaulted as you innocently go about your business. And the injustice here is off the charts. I know from my own experiences that the police have lost the plot, they seem bent on protecting offenders, appeasing them instead of upholding the law and protecting people. There's free expression for some favoured groups but not for most of the rest of us.
My grandma was French/German from a Jewish family, several of them got out before the Nazis came for them. My grandma suffered some horrible discrimination from the British after she came here before the war. So I grew up with this sort of shadow - and I'm stunned to see history repeating itself and feel so powerless to do anything.