What do you do for a cat with a fever?

My SIL claims that her cat has a fever. I'm not sure how she knows this but that's just what's she saying. She called me asking what she could do . . . she doesn't want to call the vet today

She asked if she could give Clipper a baby Asprin and I said NO . . . but I'm not sure if it would be safe or not. I'm just guessing not.