Should I tell my best friend of 13 years that i am in love with her?

Ok guys i met this girl when i was 4 and she was 3. We instantly became best friends and did absolutly everything together, wherever there was her there was me following right behind. I am 17 now and this girl is not just my best friend i am totally and completly in love with her. The only bad thing is shes engaged. none of our friends think this guy is right for her he treats her like shit all the time, but no matter how bad he treats her she seems happy. So should i tell her how i feel or just let it go? I just want to do whats best for her.