Just to add: if we assume that Team Slayer already knows about Illyria rising, then Giles could probably find out through contacts that it was Charles Gunn - Angel's trusted confidant and right-hand man - who arranged for Illyria's sarcophagus to be smuggled through Customs.

So Angel, CEO of Wolfram & Hart, is deeply involved in an attempt to raise an Elder God, and now he wants to lure Willow (who, in Giles' opinion, is incredibly powerful but also naive and prone to impulsive mistakes) into the clutches of his evil law firm?

Frankly, I think it was only Buffy's own personal remaining vestige of hope that Angel hadn't really turned evil that stopped her agreeing to send a squad of Slayers to stake him...