Just because a seat is empty, it does not mean that they open it for an upgrade. They may not have catering for it and won't upgrade someone only to say "sorry, no food for you".
I understand that this must be disappointing, but when you planned to upgrade your wife to F you would have been aware that the upgrade would not be guaranteed. You took a chance and unfortunately it did not work out. Upgrades / offers to upgrade are not a guaranteed thing; you are not entitled to that.