If they are going to DRIVE back home (or to the resort) after MGM closes, they will need to ride back to Epcot to get their car.
The BEST way to do THIS is by the MGM/Epcot BUS as it will let them out at the Epcot front gate area (where the parking lot tram is located).
Of course the OTHER option for this whole day is to park at MGM lot in the morning, take the BUS to Epcot front gate (or the BOAT to the It'l. Gateway) and then, when they are finished at MGM in the evening, their car will be in the MGM lot.
AND...
This time of year it is POSSIBLE to see the 7pm Fantasmic! at MGM, then exit, ride the boat (or WALK) to Epcot Int'l Gateway and see IllumiNations at 9pm.