Instead of wasting his money sending post cards to the neighbors to spread loshon hora, he should have gone to his Rav or his Lawyer to find out how he can collect a debt “the right way”. Something does not smell right about his tactics therefore his attempt to spread loshon horah on your neighbor will only work if YOU fall for it.
It would be a whole different situation if he KNEW YOU personally and asked you to do him a favor and speak to your neighbor b’shtika about a private financial matter between them. Since he chose to spread loshon horah he doesn’t seem like the ehrlich type and therefore there might be something not true about the story. It might just be sour grapes on his part or a completely different issue he is trying to get back at her about. in addition, there is always going to be some kind of “loss” in a grocery store. This is not something new and they account for it by building these possibilities into their prices. They also write these off on their taxes.
So as I said earlier the best thing to do in this case is to chuck it and forget it.