NO not all self injurers have bpd!!!!!!!!!!!!
bpd is often dxed because self injury is one of the symptons but also often it is misdxed just because a person cuts. yes a person can hurt themselves without being borderline.
does she see things in black n white? is she always right or worng? can she see the inbetween? does she either like someone or hate them? do her moods swing frequently? if not dont assume she is bpd just because she cuts.
u are priviledged that she let u kow about this, she must trust u, dont hassle her about it, when she is ready she will get treatment, if her cutting seems to be escalating and causing more problems/interfering with her work then sugest that she gets treatment again, but dont pressure her into it. some people can and do function very effectively in the world while still cutting. it is a coping mechanism, alleit an unhealthy one. like thunderhead said, some people drink, some people cut.