They can do a LOT mate. They can suspend military ties with China. It would hurt China MUCH more, China imports weapons only from Russia. It is sanctioned everywhere else. Apart from that, international legal law would come into effect. China can be dragged in international courts. China can/will be made to pay a HEAVY penalty. From hence on, no country would trust China, there would be a loss of credibility in the international market. China would then not be sold ToT for anything., Russia would cease to send oil etc, etc, etc.
The other countries will not then honour any legal commitment to China.
These things are very complicated. China cannot send Russian weapons to any country without prior permission. China can do nuke proliferation cuz its got its own reactors etc, so it can be done covertly and ieven if it comes out, no1 can do anything but scowl. But this is differnt.
China can use the clout it has because of its arms purchases, economy, etc, etc, etc to MAKE Russia export the weapons, but it cannot do so unilaterally.