Dear M Girl,
Some of my best friends don't like each other, so they talk about each other. I feel bad when I don't tell them and when I do tell them what they're saying. I don't want to be a bad friend, but I have know idea what to do.
I totally know where you're coming from and it's a really awkward position to be in. When you're hanging out with your BFFs and they start talking about your other friends in an unkind way, don't be shy -- tell them it upsets you! If you don't say anything, they won't know how much it bothers you. If they get mad at you, remind them that they wouldn't want to be talked about like that! It's okay if all your friends aren't BFFs with each other, but they still should be polite! Hopefully after you have this conversation, your friends will stop. Otherwise, I don't think it's your duty to tell your friends what they're saying about each other.
I hope your situation gets better and let me know how it goes!
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