My boyfriend's dad is emotionally abusive towards him.

My boyfriend asked me to help move around things in the house , and the whole time his dad was calling him stupid and making fun of him and yelling at him for doing the wrong thing. Sometimes I would laugh Because I was nervous but other times I wanted to stick up for him. And I told my boyfriend that I don't think he's stupid and he didn't deserve that.how do I deal with it? it actually upsets me
My boyfriend's dad is emotionally abusive towards him.
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