I seriously doubt that Buffy was in on the joke. That whole thing sounds more like what Giles would do as a subtrifuge to keep Angel (and later Spike) in the dark as to where the real Buffy was. As long as Angel thought that Buffy was in Italy, Giles would be able to block any kind of communication between her and the two vampires (and it's almost a certainty that Andrew told him about Spike being back as soon as he got back to London and the New Watchers' Council) who he considered "unworthy of his Slayer."
And what was the joke, you might ask? For once, someone else had made a decision affecting Angel (and later Spike) without allowing the vampire(s) or Buffy any say in the matter, something Angel did more than once.