Absolutely you can use that information. If she has an STD and he is the only person with whom she has been sexually active, then the STD can be linked back to him.
As this has apparently been investigated and dismissed once, you would be wise to make sure you have everything in order. For example, once you get the medical reports, be sure to review them thoroughly for incriminating evidence. Further, if a doctor finds credible evidence of sexual abuse, he is required to report his findings to law enforcement.
If possible, it could also be helpful to have the child examined by a mental health counselor. If she has been victimized, not only would it be helpful for her, but it could also be helpful to any criminal prosecution.
Once everything is in place, you would need to contact law enforcement. The appropriate agency would be the police department local to where the abuse took place.
Also, several customers have asked how they may direct a question to me in particular. If you specifically want me to assist you in your legal matter, just put "FOR JOSEPH" in the subject line and I will gladly pick up the question as soon as I am on-line.