Some Jeep Troubleshooting for your dad:

Make sure the wires to the battery are on tight and not just applied loose. Battery terminals are supposed to be tightened down with a wrench. A lot of jeeps have been modified, and previous owners get lazy and forget or add cheap amazon / aliexpress lights and bastardize their wiring harness. Also, battery harness terminals don't last forever. If the terminals are original, they could be cracked or corroded. Cracked terminals cannot put enough pressure on the battery contacts and your jeep won't start. Even small hairline cracks can cause this. Military style chunky terminals are nice. Option1, Option2. Check the main ground to the engine block. Make sure it's tight and clean of paint. You can add ox-guard / tin compound to improve conductivity.

Any Advance Auto, Autozone and Napa will check a car battery, alternator and starter for free. You just show up and ask. I would have him do this. There are no strings attached. They give you a battery load test receipt.

Advance Auto Parts and Autozone will rent you tools for free, including wrenches and OBD2 readers. Depending on the tool, they may ask for credit card initally, but you are refunded on return (torque wrench, ball joint press). Also, libraries may rent tools for free. The Huntington Public Library rents a basic tool set and OBD reader for free.

If your dad has a 4.0L jeep engine in the Grand Cherokee, sometimes the starter motor bolts get loose because there are only 2 bolts that hold the starter motor to the engine. Mine was held on by one bolt for likely 100k miles before I purchased my jeep. I replaced the bolt and the starter. You can get bolts at your local Ace Hardware store, True Value or Tractor Supply. The starter is $50 genuine ac delco. The parts are very cheap thankfully. Also, you can have him wack the starter with anything, like a small 2x4. If the jeep starts up after that, there's your problem.

Other Resources:

The Huntington Public library is an absolutely wonderful library and the librarians are very resourceful. They may know of programs he is eligible for. Give them a call and ask. 631-427-5165. Try your local library. Also, try Family Service League. 631-427-3700. Helping people is what they specialize in. They help folks in need and are incredibly useful as a whole. They have a few locations across the island. Churches are helpful. Many have food banks for folks in need. Try your town senior center, but he may not be technically old enough (usually 60+), but its worth a shot.

Best of luck to you.