My three year old kisses me on the lips when I drop him off at daycare every day for the whole world to see, no one has ever said it is inappropriate. My six year old how ever would just die if I tried to kiss him on the lips at school! You are absolutely right, when it is out of one of the parties comfort zone it will stop.
She should ignore the Dad it is perfectly okay to show affection in this way it is a matter of choice and it is wrong for the Dad to make any remarks , especially if the child is in earshot , your GF should ignore him, and ask him to stop making any inferance especially if the child is around.good luck ,affection like this is a good thing ...