I assume fail is NATO entering the conflict, dealing catastrophic conventional damage to Russian forces, making deep inroads into Russian territory or/and Russian society becoming extremely unstable due to the effects of the war (economical)I've asked in a separate thread, but what would constitute "fail", "serious trouble"? Feb 24th border restored, or NATO entrance into war, Moscow under threat? In first scenario, I can see China amplifying dual-use aid, intelligence sharing, ammunition. For the latter scenario, why wouldn't Russia start lobbing nukes, and China overtly giving weapons to Russia, since the world is ending anyways?
Any of those cases would trigger a significant response by China to stabilise Russia. Said significant response could involve China throwing tons of money to Russia, start serious armament aid, using coercive measures to force a diplomatic solution etc