Should I have spoke up against my supervisor?

I started working at McDonald's last night. One of the women who were training was kind of a grouch. I was in ear shot of her and another employee having a conversation. I heard her say "I'm gonna punch my kid in the throat. He skipped school today". Im pretty positive she was joking, but I feel like maybe I should have stepped in and said something. What do you think?

Should I have spoke up against my supervisor?
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