Page number please for further reference...
*Chances are I will be referencing this thread for page numbers so I do not need to search through the book during a game or tourney...
As for what my opponent, He will agree to anything that puts me to a disadvantage and never to anything that grants me an advantage unless RAW clearly supports such advantage...
Page 60 3rd paragraph final sentence tells you that the facing is determined by what facing the shot comes from. If your opponent wants to choose which facng he is shooting at ask him to show you where in the rules he is allowed to as you've just illustrated how facing is normally determined.
Page 62 final paragraph in left hand column is the only place that gvies you permission to fire at a different facing. It requires you to not be able to see the facing you are in and you can then (and only then) fire at the facing you can see but the enemy gets a 3+ cover save.