Part of the problem is that this is not your complaint but your friends and BA will want to deal with your friend (absent a specific authorisation from him) rather than you. The fact you made the booking does not give you any additional rights over the matter. He has to make the complaint or authorise you to deal with it on his behalf.

Along the process the Schengen visa likely caused problems as the UK is not part of Schengen and has it's own transit visa rules.

It would be helpful if you provided the routing involved as that could help with providing you with detailed advice.

And I'm sure you are aware but flyer talk is not connected to BA and does not have any sway over BA itself.