This is the point I got into with my atheist son-in-law Jacob over absolutes . Jake went farther to say that they did not even exist. After a few days, he and I agreed that the moral value: "service to humankind is good" is as useful and independently verifiable as the constant speed of light. IOW, he denies the existence of absolutes while assuming them and acting like they exist.
This essay agues that universals are false and useless concepts everywhere, in all cases and will so will be for now and ever shall be world without end, amen. OK, universal concepts are not absolute values-- in fact, some (like Kant lovers) dump on realism with its universals but still push an 'absolute objective idealism' (single syllable translation- it's real if it's in your mind).
These are my two biggest problems with the idealist argument -- the self contradiction of 'there aren't ever anywhere any universals', and the fact that universals ultimately gain de facto acceptance by their very detractors anyway.
If Jake is willing to invest a little patience in a question/answer discussion, you can illustrate to him how if he did not know, assume or act as if absolute values exist, he wouldn't be able to leave the house.
I've done it a few times, but some get flustered and quit before you finish the questions.
That anything is "true" is in our mind. 2+2 is only marks on a paper; looking at two apples sitting beside each other is electromagnetic waves striking a receptor.
But knowing, really knowing, that two plus two equals four is a true statement - that is an experience of mind.
So, they are right; it's real in our mind. And, yes, it could all be a dream.
However, we have to assume we can trust our experience. Else we cannot say we can "know" anything.
If someone says, "absolutes are only real if they exist in your mind," the answer is: "yes, at least if that statement exists in your mind."
We act as though absolutes exist. By definition, absolutes cannot be proved using reason/logic - else they would be conditional and therefore not absolute.
So, if absolutes can be known, we must use other means - means beyond science and philosophy. However, because it cannot be known by philosophy, philosophy cannot - logically - say it therefore does not exist.
Another method of inquiry, observation, experiment and comparison of results is required.