It is very simple. Our Holy Prophet (P.B.U.H) has taught us how to rectify our mistakes when we commit and feel guilty. Once there was a minor dispute between Abu Bakr (r.a) and Umar (r.a). Abu Bakr (r.a) got cross with some comments of Umar (r.a). Umar (r.a) felt guilty and approached Rasoolullah (P.B.U.H) and narrated the matter. Rasoolullah (P.B.U.H) called Abu Bakr (r.a) and asked Umar (r.a) to apologize. We should learn lesson from the lives of Sahabah and Seerat e Rasool (P.B.U.H) for every walk of life. You'd better get excuse from your best friend very frankly and I am sure she would excuse your mistake instantly.