Agreed with the others about staying out of it but...
Next time she tries talking badly about your boss to you... Ask her if she's spoken To the boss about how she feels. I can't speak for everyone but as a manager myself I'm totally fine with people coming to me if they have issues with the way I do things and am happy to discuss it with them and listen to their ideas and suggestions.

If I find out they're *****ing and gossiping about anyone behind their backs, whether it's myself or another team member, they're 100% in the ****.

I'd tell her you're not the one to talk to about it, if she has a problem she should tell the person she has the issue with and sort it out..