The South can hold their own. Their armed forces are very good. If this goes down ROK units will not fold like they did in 1950.
The north might do some damage early on but the south will stomp them into the modern world in short order.
In the Navy, we pulled into port in Chinhae a couple of times. The ROK Marines are NOTHING to be messed with. Total badasses.
To think they will put up or get beaten by a bunch of sawed off, malnutritioned cockroaches from the North is laughable.