Trump is not going to prison. He has a bolt hole; worst case for him is living a very comfortable exile in Saudi Arabia. He can or has stashed some money offshore, the Saudis have a history of harboring disgraced leaders and the prince likes making Biden, the Democrats, and westerners in general look weak. Trump being rapey, power mad, and corrupt also makes him someone the prince can relate to.
I can't imagine the MBS putting up with Trump any more than he has to.
(Is he going to show Trump how to behead people?)