This answer suggests that should Russia use a nuclear strike in Ukraine, even China would condemn it:
By using weapons of mass destruction Russia would be condemned not just by the West, but also the most countries which are friendly to Russia, like China, would condemn such action as well.
CBS News mentions the relunctance of the Chinese officials to condemn what very likely represents war crimes in the Ukraine:
Beijing — China on Wednesday said images of civilian deaths in the Ukrainian town of Bucha are "deeply disturbing" but that no blame should be apportioned until all facts are known.
I am wondering if any Chinese officials ever mentioned a "red line" in the Russian invasion, that is what Russian forces actions could trigger a clear Chinese condemnation.