Is it socially acceptable to tell someone if he or she is being egregiously rude to a store worker?

I was in McDonalds and this older gentleman (in his 80s) was being extremely rude to the cashier woman (young hispanic woman). He was trying to order a biscuit along with his breakfast, but the woman said he couldn’t do that. He kept complaining he couldn’t understand her english to a point she was about to get cry. Her english was fine and was just too old to understand he couldn’t order just a biscuit at McDonalds. I told the cashier I was sorry for her, which prompted another lady to say the same thing. However, I wanted to tell him off since the cashier lady couldn’t or else she would get in trouble, but was timid.