Are revealed preferences normative or positive notion?
However, I encountered an opinion that since all preferences derive from underlaying values they are all strictly normative and considering them positive even in the strict sense of revealed preference is a fallacy.


There's a logical inconsistency here which might help see why it's not a good way of looking at the issue.  What is the key element that is purportedly revealed by dividing the universe into "things normative" and "things positive"?  I would suggest that it is determinism - matters of a positive nature correlate by way of fully deterministic, structurally-modelable processes that are discoverable through repeated experiment.  Think physics - laws guide observables, observables reveal the laws, the laws can be used to predict future observables.  If a tree falls in a forest and no one is around to hear it, physics tells us that it still makes a sound.

Matters of a normative nature correlate only to the extent that a system of "norms" has been set up to enforce them.  Think ethics - morals guide behaviour, behaviour reveals morals, but can morals - absent a normative framework - be used to predict future behaviour?  No - because morals themselves can change, because agents are capable of deception, and because agents may not even have perfect knowledge of their own preferences. Crucially, morals alone are no guaranteed predictor of unexamined human behaviour.

So, I'd argue that your friend's opinion is logically inconsistent because it is a contradiction: in order for it to be true that all preferences derive from underlying values, it must be true that values influence behaviour in a deterministic way according to underlying laws that are, in principle, discoverable through experiment.  That's a pretty weighty claim, in my opinion not at all borne out by the preponderance of evidence - but if it were true, it would mean preferences are in fact positive!

Whether preferences are normative or positive then depends on the assumptions about the agent.  We usually want them to be positive so that we can math about them, and so we make a whole bunch of assumptions about homo economicus that - if true - support that assumption.  But of course, we know that homo economicus is a fiction. So we might suspect that our theory on preferences is at best a useful benchmark to provide context for understanding how people actually behave.