Cynthia was Stanley's nurse, and a relationship budded between them. With Stanley's wife Terri out of town, Stanley and Cynthia went out to a club called Tinks, where they were seen by the summer interns Megan, Eric, and Maurie. Michael learns that Stanley is having an affair, and accidentally tells Terri over the phone. Terri and Stanley have broken up since.
Cynthia was Stanley's guest at Jim and Pam's wedding. Cynthia and Stanley both danced during the JK Wedding Dance parody.
It is possible the two married without telling anybody. This is able to be theorized by Stanley checking into a room under Mr. and Mrs. Hudson in Niagara.
In the deleted scenes of the episode Garden Party, Stanely broke-up with Cynthia at the garden party because he said she wouldn't make a big scene if they were in public. But she did.