[Sunlight in Pylea doesn't harm vampires.]
Angel: Can everybody just notice how much fire I'm not on?
Gunn: As evil blood-sucking vampires go, how would you rate Angelus?
Wesley: Historically, as bad as they come. Especially when he was with his sire, Darla.
Cordelia: We're researching her now to see if she has some kind of resurrection powers. Maybe she's a vampire cat with nine lives, or something!
Gunn: So he and Darla together, bad combo?
Wesley: They rampaged through half the known world until Angel got his soul.
Cordelia: Imagine Bonnie and Clyde if they had 150 years to get it right.
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.