Start early, stay late, and do all three. Let them know they're coming. DW might take into consideration their situation. If they're staying in a disney resort or hotel that night they can take advantage of the early opening. If it passes the test there's also the extra evening hours. And they may even get lucky enough to have an E-ticket night, although I hear that's been discontinued indefinitely.
I don't think there is such a thing as a one day park hopper, but they can get a three day one park per day and burn it up, or if they think they'll be back again get a 4 day park hopper and save the other three days.
From the TTC they can take a paddle boat to MK, monorail from MK to Epcot (via TTC), and then take another boat from Epcot to MGM. You should be able to catch a bus back to TTC from MGM.
Breakfast at Cinderella's Royal Table or the Crystal Pallace if they like Pooh, lunch in one of the countries of Epcot (they each have their own lillte special flare), and dinner at the Hollywood Brown Derby followed by Fantasmic.
In theory they should be able to hit at least two major attractions per park in the busy season. If they are there in the off season they should be able to do much much more. And if they're in thier mid twenties they should have the energy to run this marathon.
Depending on what time of year and what day of the week they will be there Eeyore2003's plan might work out better.