If I'm giving flowers to someone as a birthday present, I like to give an actual plant instead of cut flowers.
I enjoy the thought of my flowers continuing to grow and bringing a smile to my friend's face each time she/he sees them in her/his garden.
Here is my list of 5 great flowers for birthdays.
Of course, I'm sure your friend would also enjoy a lovely bouquet with any of the extremely fragrant flowers mentioned below.
I've seen tulip bulbs for sale even in the dead of winter, which makes it easy for me to create something fun for friends with winter birthdays.
I'll place 4 or 5 tulip bulbs in a brightly decorated planter filled with potting soil and then give the container to my friend as a gift.
He/she gets to enjoy watching their gift grow and bloom as time goes by.