Well, first of all, I seriously doubt that Willow would have refused to help if asked. If you look back at that episode and Angel's phone conversation with Giles, the Watcher refused to either relay the message to Willow or give Angel any help in even allowing direct conversation between the two (It's always struck me as strange that, once Angel knew that Willow was in Rio he didn't just use the W&H facilities to contact Willow directly, and to hell with Giles, but that's not what we're discussing here, so...). I've said it before: I believe that Giles purposely blocked communication between Angel and any of the Scoobies or Buffy herself, or even Faith. Almost certainly Andrew reported the fact that Spike was back to Giles as soon as he returned to England, and that would have doubled his reasons for keeping any such information from Buffy. Giles at the least distrusted, and at the most hated both vampires, and more than anything else he wanted to keep them away from Buffy "for her own good," which seems to be a motivation greatly overused in the Buffyverse. Someone is ALWAYS trying to protect someone else from some unpleasant situation, whether real of imagined, and it almost always ends up doing more harm than good.
So, anyhow, back to the question: Giles refused to help because he distrusted Wolfram and Hart, and had a thorough disliking of Angel and Spike. Willow didn't help simply because she never was informed of the situation. If she had known, considering the almost instant way she and Fred hit it off, she would have, literally, been there in a flash.
