"Your itinerary is about a straight forward as it gets."
No it's not. He has a return ticket London/Shanghai on BA, and he has a separate return Shanghai/HKG on China Eastern. Nesting a ticket like that is not as "straight forward as it gets."
Like I said, he has to accept there is a risk that that itinerary will raise eyebrows at check-in and he'll have to deal with the check-in clerk. As the few posts in the sticky demonstrate, being right is not a guaranty you'll be getting on the plane.
Okay. MU-MU would certainly be
even more straight forward, but anyone with an IQ >100 could figure out that BA-MU is also compliant.