I have done this itinerary many times, it is fairly hassle-free if you don't mind crowds, construction, and a couple of long walks.

Arrival from ICN is in TBIT, which always has been a mess and likely will always be a mess. They may arrive at the remote stand and be bussed to the terminal - evening is a busy time as all the long-haul flights to Asia load up.

Re-check after customs in TBIT will get their bags over to the DL terminal (provided KE tagged them to ATL - never been a problem in my experience, even when originating from a KE outstation in China). Depending on the status of construction this may be an actual counter or a couple of skycap types with a cart - since they've got time in their connection I wouldn't worry about bags making the transfer.

Next they get to sample some of the lovely LA weather during the walk over to the DL terminal (T5, exit TBIT and turn right, keep walking past the AA terminal). There is a 'connections' bus (Shuttle A) but it's not worth the wait and I usually can stand to do some walking after a long flight.

KE *should* have been able to issue boarding passes for the onward DL flight when originating in ICN, so they can skip the check-in lines and head straight for security. While it will be good enough for the TSA, KE BPs won't scan at DL gates, so they should get DL BPs from the gate agent or kiosks in the gate area (or Sky Club) - beats holding up the line during boarding.

Overall it isn't a terribly difficult process, KE and DL have been partners for years and have gotten most of the bugs worked out. Although TBIT is a zoo in the evening, follow the signs and everything should work out fine.