The best counter to moral relativism is still the quip “is that so?” delivered with the appropriate raised eye-brow.
Any sufficient response to that query must consist of a positive statement of an absolute value which proves moral relativism to be a fraud at best.
While academics and other invested relativists insist that such verbal slaughter falls far short of fully discrediting their preferred viewpoint, they must first dismiss the truism that any philosophy that is internally inconsistent cannot be truth.
But just in case one needs more proof, consider the idea of FGM.
Female Genital Mutilation, or Female Circumcision, is a barbarous practice found exclusively among cultures whose religions require extreme subservience of their women.
The ugly process ensures women will derive no pleasure from sex with the supposed goal of guaranteeing the servitude of that woman to her man in all matters of sex and children.
The side effects are pain during intercourse, increased pain and damage during childbirth, and increased chance of infection.
There is no way this practice is moral. It produces no realistic, practical, or natural good for the woman, and who could argue successfully such enslavement of women is good for the men?
In America, we can fix this. We can remove the terrible effects of this mutilation. We can restore pleasurable sensations during sex and lessen the pain of childbirth.
That is a moral good.
Now, if some still want to argue that all cultures and individuals can find their own good which may or may not also be good for someone else, let them defend FGM. Let them defend the pain and the suffering. Let them say that action of mutilation is the same, morally, as the American action of restoration and healing.