I'm going to tell my wife's best friend to not gossip about their other friend
Why do you want her to stop gossiping?
Because it's pissing me off.
Gossiping can be rude sometimes but you shouldn't try to control the conversation. It will make people think that you are rude.
She needs to stop doing it.
You shouldn't try to control what your wife's friend says. It doesn't sound like that big of a deal, so why not just go along with the conversation?