It depends on if you’re arguing with someone who is arguing out of spite or intelligence.
If they’re arguing out of spite, the short answer is: you can’t.
I’m not saying that relativism is more “right” just that… well let me answer your question with a question.
“Can you tell me something so simple that there is nobody so stupid they can’t understand it?” Every time you think you have an idiot-proof argument, the universe will invent a better idiot.
Relating that to relativism, an obnoxious relativist can merely say “well that’s your opinion.” Again, not saying that it is a more correct argument, just that it’s difficult to make headway against.
If they’re arguing out of intelligence, then here is what I would attempt.
You can’t argue that specific actions are good or evil since it really is motivation that makes them so (i.e. donating money to charity for charity’s sake is a morally better action than donating the same quantity of money for the sake of a tax write-off).
But here is the argument I would use. The definition of a “good” action is one done to make the world better from the perspective of the person doing it.
Note, that is not saying that others cannot weigh in on that person’s assessment of “good.;” from a lioness’ perspective, killing me to feed to her cubs is a morally good action, but that doesn’t mean that I will let her do it.
An evil act desires to make the world worse. This can be as petty as “I have to get up at 5am, so I’m going to be loud and wake my wife up because if I don’t get to sleep in, neither should she!” Or it can be of a larger scale, “My life sucks, let’s go beat some black people do death!”
Now, your relativist friend may counter that the people who are hurting other people have a view that they are making the world better. This argument serves to do two things:
1.) It states that they acknowledge that all people see “making the world better” as a moral end to head towards (and hence secures that argument), or why would these people rationalize it as such?
2.) It is on a weak footing because the argument is so open to criticism. If these people were trying to make the world better (say by taking what they view as an undesirable race away), they would be open to more effective or more constructive methodologies.
The third option is just to be annoying.
You: “There is an absolute truth!”
Other: “No, everything is relative.”
You: “The statement ‘everything is relative’ is an absolute statement, so if that’s true, then that must be an absolute truth, and if it’s relative, than there must be exceptions to it, which means that there must be absolute truths.”
It’s a circular argument that doesn’t actually mean anything, but if they’re just being obnoxious, this is a way to beat them at that game.