Ask them to provide a mainstream media source, ctrl + f zenz (like 95% on articles on BBC and the like use him as a source) hit them with the "Do you really think a German evangelical member of the victims of communism foundation on a self described 'mission' who writes books such as Worthy to Escape: Why All Believers Will Not Be Raptured Before the Tribulation is a reliable source?"

If their specifically going for 'genocide' you can use the UN office of human rights report that found no evidence of such or mass killings, but does allege other human rights abuses but assuming you arguing with a burger can counter Guantanamo/extraordinary rendition/mass surveillance of Muslims/Abu Ghraib/ and similar human rights abuses perpetrated by the US and it's allies https://www.ohchr.org/en/documents/country-reports/ohchr-assessment-human-rights-concerns-xinjiang-uyghur-autonomous-region