Dear "Friend":

Just a little note: if you say mean things about someone else that I love, I will probably think a little less of you. Maybe that's obvious. But here are some clarifications that may not be:
1. I will like you less than I did even if, in general, I like you more than the person you're badmouthing. And, I will probably end up liking them better.
2. I will associate you more than the person with any bad things you might tell me about them - even if they are true.
3. I am probably aware of the fact of whether or not you like the person.
a) if you actually like them (or love them) I probably will judge your motives less harshly,
b) however if you dislike them, it will make me defensive on their behalf.
4. It kind of makes me want to hear them say bad things about you, and if they do, I probably won't be grumpy, as it will seem deserved. That means this formula won't apply to them.
Good luck,
H.