Is it bad to call yourself a slur?

My moms coworker is Mexican, and I overheard her having a conversation with my mom about something that happened at work. This new employee, though I hate to say something like this, thinks that everyone hates her because she's black, when actually it's because she thinks everything should be handed to her and no one can do anything she moderately doesn't like or else they're being racist towards her.

For example, my mom and some of the other coworkers invited her out to lunch after work. She said no, which is fine, but obviously the group just went without her anyways... The girl decided to go to HR and tell them that they all went out to lunch and never invited her because she's black??? ...I feel like even if they hadn't invited her, it would make no sense for her to get mad and tell HR anyways because why are they obligated to invite her places?

Back to the point of this post; so my moms friend likes to refer to herself as a "be*ner" jokingly. She doesn't go around screaming this at work, and she doesn't call other people slurs or insult them for their race or anything, all she does is call herself one. She was in a conversation with my mom and the other woman, and she called herself a be*ner. The other woman got mad, and told her that no one should say slurs even if they are directed towards them, and called her racist for it.

I just don't get it. She didn't insult anyone, and she didn't even act like it was bad to be Mexican. All she did was call herself a slur that is literally directed towards her race. Is it bad to call yourself a slur?