If he's always in paid business class status won't add much.
If the lifetime statuses are not in play, I guess the best thing to look at would be which airline offers more value for its miles and upper tier status (e.g., would he be in range for CK?)
Does he envision doing some leisure travel on AA especially with non-status companions with him? That would steer towards AA too.
Is the move temporary for 1-2 years before going back to London? Would steer towards BA on if so.
I think any time without status on AA during the transition wouldn't matter if on paid business anyway, and if in business on expensive long haul tickets status will come fast.