Can dealing with condescending comments damage an adult's self-esteem?

My sister has recently gotten together with a really good guy. He’s sweet and smart, and she says he takes great care of her.

Thing is: When he gets angry, he says questionable things. She only told me about this yesterday, but he has apparently called her “a piece of shit” and “a psycho bitch”.

Now, she’s not innocent. She can be pretty infuriating (I should know). When they’re done fighting he’s apologetic. But still, I’m worried. He doesn’t make any attempts to prevent things like this from happening in the future, she says.