1988 Trans Am

My son bought a 1988 Pontiac Trans Am
8 cyl with 84,000 miles. 5.0 litre TBI engine 4 speed automatic form someone on ebay. The guy told him he could pick it up and he did. He drove it about 100 miles home. The oil pressure gauge did not work. He got it home, it ran fine. He shut it off. He decide to swap out his radio and was sitting with the car started it and disconnected his radio. The car ran for about 20 minutes after disconnecting the radio and then it died. He tried to restart it and it would not crank. Clicked a few times but nothing. He started to check things and found there was no oil registering on the dip stick at all. He added oil still no crank. We tapped the started and still not crank. Replace the battery with a new one. Still nothing. The 3rd time he tried to start it there isn't even any clicking. Just nothing.
He said when the car died it just died quietly no noise no bang. No overheating. Just shut off.
I am worried he might have seize his motor. But if that were the case wouldn't you know it seized? Some noise that didn't sound right accuring?
I guess he should have checked the oil but he figured the guy said it was ready to go. And would have said something if it needed oil. UGH!
I was told you can try to turn the crank by hand to see if it is seized. Clockwise I believe.
how far should it turn and do you have to remove anything when trying this? Should the car be in gear?
Could it just be the starter? But the starter shouldn't make the car die correct?

can someone tell me how to figure out what the problem is?

thank you in advance!