The Cautionary Tale Of Numero Cinco - S5-E6
Revealing mistake: When Angel says to Wesley that "Fred has got something," and then walks out of the room, Angel's reflection is visible in a window pane in front of him. It's very brief, but it's there. (00:31:05)
Answer: Everybody including Angel saw her. After helping them and then being left alone to talk to Angel, Cordelia says that the Powers That Be let her come back one more time to help them. After Angel gets a call saying that she died, he realises that it was her spirit that was helping them the entire time.