>>And how will the U.S. respond to China if this transfer goes forward?
That's the key question. Whether it's at all advantageous for China to do so to the side (spoiler: it's not), China's economy is in a shambles; they couldn't weather the sanctions that would follow as a response, particularly if those include access to SWIFT.
If they provide any lethal support, it will be things not easily traced back to them - e.g. small arms ammo - yes, I know brass is traceable if you know what you're looking at, but those marks can be obscured, or RF ammo marks 'counterfeited'. If China is really good at anything, it's counterfeiting...
But, back to why it might do that. Russia is already the weaker partner, well on its way to becoming a client state. It's already selling China all the energy it can (limited by which pipelines go where). Having a grateful Putin government gets them nothing. And they probably don't see him in power long enough to matter, and any promises from him worth, well we all know what those are worth. We're back to the old saw: "nations don't have friends; they have allies".