Well, this line of reasoning will end up going Godwin in a hurry.
But for entertainment value, to make a different argument against moral relativism: A higher proportion of people from Qatar would like to e.g. live in Europe than the proportion of Europeans who would like to live in Qatar, and vastly higher if we take relative social standing into account.
I'm sure there are plenty of ethical questions where a hypothetical, neutral outside observer would have trouble choosing. There are more ethical question where the neutral outside observer would easily be able to discriminate what cultural traits they preferred, while at the same time understanding why those sub-optimal cultural traits came to be and play a role in that particular society. Imagine sexual mores in a society with no birth control, lots of hard labor and the preference for peaceful succession due to early accumulation of capital (farms), for instance. It's easy to understand what role traditional and restrictive morals has in such a society, while at the same time acknowledging that living within the morals of such society would be a giant step down in quality of life.
So that's the complex argument. But that all ethics is generally speaking relative from a human perspective, can be trivially disproven by pointing out ethical standards that almost all societies except those that have them will find repugnant. Consider e.g. genital mutilation or institutionalized child abuse.