This has probably been done to death, but here goes!
I'm flying long-haul with BA with a friend who has a life-threatening allergy to lemons. He is convinced that he can just ask the airline to not serve and lemon/lemon products, and get the airline to tell the other passengers not to eat any lemon/lemon-containing stuff.
How right is he? I can't imagine the cabin crew could ban everyone from eating lemons, only ask them if they would mind not doing so. He isn't allowed an epi-pen, so any reaction would probably mean landing ASAP.
He has contacted BA, but had no response (he claims).