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.