Desk-pilot:
You might want to talk to your friend and give him a few friendly pointers.
Another Captain I know and keen sailor is contemplating training as a liner Captain with P&O or whatever because he'd see the world and enjoy the status and tradition - fine dining etc that airline Captains are nowadays denied.
To train from scratch he'd need to go to college, pass the necessary exams, get the minimum qualifications and start at the bottom. His 'Captain' qualification from flying would do nothing for him at sea. He'd be joining many other guys working their way up the ladder. Unfortunately, money can't buy a Master Mariners ticket. Years at sea and experience are required and the chance of ending up as a P&O Liner Master, well, pretty much the same odds as becoming a premiership footballer.