In y experience, If they rent a car from one of the many in-terminal options, they will be much faster than ME, and have more options. If they don't want to do that, they could also get a taxi very easily. (but would then have to go to hotel to drop luggage at bell services)
If their flight is on time and all, then they could probably get to CP by 5pm, but may end up missing if ME is very slow, or their flight is delayed. ME often makes multiple stops! The stops are very slow as guests get luggage.
We often don't book ADR's our first day because of the high risk of flight delays. I would also consider how well they know WDW. another small factor is the timing of the in airport shuttle.
If they get their own car, they could go directly to MK parking, and leave luggage in trunk. Then they will have a car all week = big plus.
One trip a family member got to MK 45minutes after landing using ME! Another trip 20min flight delay+ ME meant arriving at my hotel nearly 4 hours from scheduled landing time.
I-4 also gets traffic, but should be okay when they are commuting. We've seen it backed up at 3:30pm.
I would not spend all day on the phone stressing, but would have them contact you via text when they land and chart their progress.