Lets b e a little neutral here. Yes its not good to forget a birthday but it happens. Lets not go overboard. As for calling her stroppy. Thats a strange choice of word isn't it. Of course that's not good but I'd like to know more about how she reminded him. Was she indignant, did she nag,. If so the question is how to tell somebody they forgot an important date.

Had I been her master on realizing I would arrange something special and tell her I don't celebrate birthdays, its expected, its not a surprise. I celebrate the 5th day after the birthday.