China is their only ally and major trade partner. In the past, China ignored trade sanctions against N. Korea and freely traded with them. They are finally honoring trade sanctions. N. Korea is communist. China doesn’t want a communist country to fail. It’ll give 1B people ideas that communism isn’t that good.
Why would China worry about N Korea having nuclear weapons? N Korea would be stupid to nuke China. China is literally their only ally.
Again, what has China done about the problem so far?