cowardice?...

"no, i'm not telling him that! i don't want to see him cry in front of me, asking me what he has done wrong or if i could give him another chance... no way!"
some girls tell that. or it might be vice-versa, but let's take this example...

chicks are afraid of telling the guy they once loved and thought he was the perfect guy for them, that it's over.

sometimes, this truly can be a difficult thing, but think of the poor dude. how could he feel in this situation? when he tries to call you and you hang up? when he comes to pick you up from school, and you're leaving through the other entrance, just not bump into him?...

and the rough part... the friend who is in between. who is the best friend of the chick, and is becoming a good friend with the guy. what about her? don't you think that the guy will ask her about you? and what would your best friend tell him, when she sees him with tears in his eyes, saying that he loves you and that he wants you back? that's not a nice situation, especially for the person who is your "best friend". she is, if she helps you, but what about you?

so, don't make the mistake to put your own on the shoulders of the one that cares about you (your best friend). and don't hurt the feelings of the person that loves you. you should try to explain your feelings. it might be hard, and people might be very angry with you, but they will finally understand the decision you took.