Buffy, I think, would have been more inclined to accept and trust Illyria, given her record of doing the same thing with both Angel and Spike, though her memories of Glory might make that a bit more difficult.
Xander would probably fall head over heels in love with her.
Giles is suspicious by nature. He would, I believe, be the hardest one to convince that she was an ally, but in the end it wouldn't really matter what he thought, because
Willow, the only one of the group to have ever met Fred, upon finding out that Giles had refused to allow Angel to contact her for help, would have turned Giles into a rat, then resurrected Miss Kitty Fantastico and locked them in a dark closet together.