Seeking information related to a rape case: how viable is prosecution in this situation?

My girlfriend was raped repeatedly by her stepfather from some age pre-puberty (does not remember exact age due to trauma-related memory loss) until college age. Her mother did not take her reports seriously until she suffered a nervous breakdown and suicide attempt, after which she was entered into inpatient mental health care. Her mother divorced her stepfather, but refused to support her daughter in prosecution, saying that she "couldn't take it," and has concealed the reasons for the divorce from nearly all her relations. The divorce occurred roughly seven or eight years ago, and the crime was never prosecuted. However, by my understanding of New York law, the crime constitutes rape in the first degree, which has no statute of limitations. If we were to choose to prosecute, how viable is the case likely to be? There should no longer be any biological evidence of the crime, but my girlfriend's inpatient treatment, ongoing therapy, and PTSD diagnosis are all matters of record.