Power Play - S5-E21
Factual error: Spike suggests that Illyria and Drogyn play Crash Bandicoot. They appear to be playing on an original Xbox console. The episode was broadcast on May 12th 2004, meaning that the only Crash Bandicoot game they could possibly be playing would be The Wrath of Cortex which came out on April 5th 2002 in North America because every other Crash Bandicoot game that came out before that was only on the Sony PlayStation (with the exception of Crash Nitro Cart, but they're not playing that one). Both Illyria and Drogyn appear to be playing the game. However, The Wrath of Cortex was a single-player only game. They're not playing Crash Twinsanity (available for the Xbox) either as that didn't come out in North America until October 1st 2004 and it too was a single-player only game.
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.