I suppose it depends on what the person did to offend your mother. But if your mother over reacted and created a problem for someone who may not have deserve it, then yes, it would be appropriate to chastise your mother for it.
Everyone makes mistakes and it is important not to judge them too harshly or create problems for them simply out of pique. Maybe your mother made a mistake and should not be judged too harshly either. But if she did over react and you really feel she created a problem for the person far in excess of what they deserved, you should talk to her about it and, if you feel it is appropriate, strongly recommend that she go back to the store and tell the management that she was in error.
We each have a responsibility to behave ethically.
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Never let the weeds get higher than the garden,
Always keep a sapphire in your mind.
(Tom Waits "Get Behind the Mule")