I have this friend would youcall her a back stabber?

well i have been her friend since i waz 2 1/2 and i am 12 now she has bennn flurt in w/ my boy friend BAD like puttin stuff down his shirt and like follow in him around and stuff she has alwase been weard like she eats paper and stuff & she is makin me mad and she has a boy friend so she has a boy friend and she is flurtin' w/ mine what should i do and do you think she is a back stabber>????!!