There's a big misconception about the theory of relativity in physics. It does NOT imply that absolutes don't exist. It just implies that space isn't an absolute and time isn't an absolute. Also, very importantly, it implies that the speed of light measured by an inertial observer in a vacuum IS an absolute. There are other absolutes also, but I don't know them all that well and don't feel like going into them here. Ask AI if you want to know them. The point is NOT EVERYTHING IS RELATIVE even in this theory. There's at least one thing that isn't.
But some people don't want to look into things that deeply. Instead they hastily develop a class of thought based on the faulty premise that because certain things we previously believed to be absolute turned out otherwise, then nothing is absolute. This is false.
Whether morality has no absolutes is technically still up for debate I guess, but moral relativists can't really use the scientific theory of relativity to back their claim, because that theory DOES claim absolutes, even if it sounds like it doesn't.