Maybe im one crude little wanker, but
I would put the note on her desk, strictly about HER habits and how SHE has been seen NOT washing her hands.
then, smear a little
chocolate on the corner ,
or the back...
I think not washing your hands while you work in a small office with many people is disrespectful. Do it at home, on your own time, but be considerate of other peoples hygiene.
Everyone has to use that bathroom, door, phone, elevator button etc.
but, im neurotic I guess.
Medically speaking, you are safe in a bathroom if EVERY person didnt wash thier hands before touching the door handle.
Doesnt make it any better to know that though...