What do you do when your best friend cheats and has become top 10 in class of 250?
You say that several students are cheating. This is actually good in the sense that you maybe can avoid specifically naming your friend.

One option is to contact a relevant faculty member (ideally one that you know and trust) and say that you know that several of the top graders are cheating. A second option is to do this anonymously.

The advantage of doing this anonymously is obviously that people will not know that you reported this. The disadvantage is that it may be less credible.

BTW your friend is completely incorrect. It definitely does affect you, since the ranking and the prizes can affect your future career. If she can't understand that, then she perhaps isn't such a good friend.