At that point you just say “that would be well and good, but you’re saying the onus is on me to prove a negative. In this situation you’re the only one who can possibly prove their claim and I’d like you to do it.”

That's not how burden of proof works. The burden of proof principally falls on the person making the positive assertion because you can't prove a negative (in most cases). If I say that everything heavier than air falls to the ground, that is the common view (and technically false) but it still requires me to at least give some examples to establish a basis for believing the positive claim (dropping a pen or whatever). With that established, it falls on the other person to provide counterexamples (planes, helicopters, etc.)

It's not just "the common view" for many reasons, not the least of which being that "the common view" changes in different places and over time. The Uyghur genocide conspiracy theory is popular in the anglosphere and Europe, but most of the world does not agree with it.

If your friend is an atheist, the easy example is that at some points in history the existence of an Abrahamic God was the clear local consensus in many parts of the world, but that does not mean it would fall to the atheist to disprove God without any real standard of evidence being established. It would still be up to the theist to provide a basis for the atheist to then refute.

Your friend is just being lazy or a coward, if they understand that their view is the "default" position, it should be no trouble to them to produce supporting evidence. Surely they wouldn't just believe such a dramatic claim without evidence, right? :thonk:

Edit: the Holocaust is real and the common view, but it still would be up to me to point to evidence (e.g. the camps, photos of survivors, etc.) when faced with a denialist.