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Good or not, it is considered bad social graces to speak ill of the dead. This prohibition is enforced for many years. Anyone that speaks ill of the dead is showing a bad upbringing and bringing discredit upon themselves. You should defend but just realize you are defending against someone with no manners.
People will always give their opinions on others, but they should also have the tact to limit said opinions before they become disparaging remarks about someone whether they be dead or alive.
If someone is being so tactless as to speak poorly of your dead mother to you, your best bet would be voice your displeasure with them and then just ignore them, because I doubt they have the moral compass needed to understand what they're doing is distasteful and wrong.
Take care.
Perhaps my mail box will be spammed for the opinion piece I have written above, but gee whiz, some people are so rude these days.
But, people should still keep their opinions of others peoples dead relatives to themselves, out of respect to the living person.
I agree with you OlderCPerson wholeheartedly that we should not speak ill of the dead, but I would like to differ in that our refrain is not only due to respect of the living but to the dead as well. A person had served their karma whilst alive and paid all their dues when dead. If the deceased owed others and could not repay whilst alive, the debt would be repaid in the next life, in the next karma. I am just a humble soul and not wise enough to pass judgment on another. Let God perform judgment on his creations when arrived at Death's door.
I agree with you WuWei in your retort to the hypothetical critic who insulted your mother. I would add that: "if you feel comfortable with your disparagement of my dead parent, then you would not mind if I criticise your flipping irresponsible parent that has brought you up to be an insolent and disrespectful cow!". Then quickly exit stage left lest they throw a rock at you. He he he. I also concur that you should avoid the creep, and practise social distancing when approaching his/her proximity lest you catch their insolence like catching the corona.