When people say something like, "you're just being oversensitive, I meant it as a joke/compliment", I find that they are simply trying to justify what they said, instead of thinking about how they act and speak impacts other people. If someone says that you have hurt or offended them, listen to them - you do not get to say how someone else feels. On that note, though, I don't really think that Person A can tell Person B their language is offensive to Person C - unless it is something that A & C have discussed before, and Person A is acting as an ally. Sometimes it's really hard to speak up in the moment when someone has just said something really insensitive!