China views the US as an adversary. They would very much like to reduce America's sphere of influence and are cognizant of the fact that we might end up in a war in the medium term future. The US bleeding immense amounts of money and military assets in Iran is great for China's relative strength; it's in their interest to increase those costs in ways that don't escalate immediate tensions with America. Sharing targeting and other intel is one of the more effective ways of doing that.