A Friend living in South Korea says that North Korea is a much bigger threat to China than to the U.S. or other countries

This is completely true by the way. The paranoia over North Korea blinds people in the U.S. and Europe and Canada and even Australia and New Zealand from likely seeing this clearly.

Why does China support (tacitly and very very carefully) North Korea.

1. They don't want their Chinese citizens (the average person not in the elite class of China) to see how well South Koreans are doing with democracy and capitalism. In other words they don't want to see Samsung, Kia and all the other big South Korean companies and businesses doing so well.

Instead they want average Chinese to see sad little North Koreans who are having terrible problems starving to death because China then seems a whole lot better to Chinese when they see or meet North Koreans.

2. They are scared to death that their little North Korean friends are going to start a nuclear war with the U.S. or Japan or South Korea which inevitably would bring U.S. and European nuclear weapons against North Korea which might leave North Korea and that section of China next to North Korea then uninhabitable.

3rd they are worried in China that Kim Jong Un has no loyalty at all to China for some reason (maybe megalomania) or some other form of manic mental disorder and nuclear weapons will either be used on China or radiate the whole area (including China).

Even now, the missiles North Korea is sending into the sea likely are killing fish with chemicals or radiation which might make these fish unfit to eat for Chinese people. So, likely Chinese people are already dying indirectly from being poisoned by what North Korea is doing.

And this likely is also true of North Koreans dying from radiated fish, Japanese dying from radiated fish, South Koreans dying from radiated fish and Japanese and other countries likely have people
dying from poisoned or radiated fish too. (All of them from either poisoned or radiated fish.)

So, for all these reasons and more China has much more to worry about from North Korea
than the U.S. , South Korea or any other country.

The other point that my friend made is that people in North and South Korea are all related in the same way people in Eastern and Western Germany were all related before the Berlin Wall came down. Only most of them haven't seen their relatives much or at all since around 1950 or thereabouts.

So, because of all their estranged relatives they don't want to kill each other because they are family in the end (all the common people in both countries).

However, then another way to look at this our own civil War from 1860 to around 1865 where relatives and friends killed each other then too. So, this is a very touchy business like it was and is still for us from what happened during the Civil War still.