The captain should NOT take over in the event of a stall normally (maybe different on a training flight if the trainee fails to react). The F/O is PIC/US and is fully trained and should be expected to react correctly to a stall warning
You must be kidding! Meekly leaving it to the F/O simply because it is "his leg" is a complete abrogation of the captain's responsibility for the safe conduct of the flight. And if the F/O stuffed it up, how long should the captain sit on his hands before he decides to take over control of a fully stalled 737. At the altitude that 737 stalled, it takes swift and concise action and it would take a competent captain barely a nano-second to take control. The blind faith that the F/O is a highly experienced eminently skilful copilot leads to complacency and complacency can kill. The claim that any F/O is "fully trained" has zilch to do with situation. The captain, too is presumably "fully trained".
The buck stops at the captain - not at the first officer.