The ACT is another college entrance exam. As far as her taking it, it depends what college she is interested in going to and which test they want. When I was in high school, some colleges wanted ACT scores and some wanted SAT scores. Most schools that wanted SAT scores at that time were East or West Coast schools. Some took either and alot of Midwestern (where I'm from) schools wanted the ACT. I and most of my friends only took the ACT.
That was 17 years ago though. I think more schools now are accepting either, but I don't know for sure. I'm sure her school guidance counselor or principal would probably know.