My last post focused on your drive. I also have some thoughts about the hike.
If they want to spend the night at the bottom they will need reservations at Phantom Ranch. These reservations are hard to get and must be made a year in advance. Check into when reservations open and try to get in right away. In an hour or two they are filled up. I have tried once or twice and have never gotten in.
When a person takes a mountain hike, they are usually hiking up when they are fresh and downhill when they are tired. Hiking in the Canyon, the last part is uphill.
All the best to them,
nacl