First, that's not a thing. A rape kit can't prove no sexual assault occurred.

Second, all kinds of shady stuff happened:

This is one of the police officers she found, according to Sports Illustrated:

Sgt. Jerry Blash, the Milledgeville police officer who wrote the criminal report, has resigned from the department in the midst of an investigation into comments he made to Barravecchio that night. According to Barravecchio, Blash described Roethlisberger's accuser as "this drunken [REDACTED]" and said, "This pisses me off, that women can do this." Sources say Blash—who had posed for photos with Roethlisberger and several other Milledgeville cops earlier in the evening—was dismissive about the sexual-assault claim when the accuser and her friends approached him that night. ... Milledgeville police failed to secure the crime scene, and Roethlisberger was never interviewed in any formal fashion that night or in the ensuing days.

So Roethlisberger left on his own while the alleged victim went to the hospital for a rape kit. Video from the club that night later disappeared, and though doctors found male DNA on the girl in question, investigators eventually withdrew their request for a Roethlisberger DNA sample.

In the end, Roethlisberger's accuser elected not to continue with the case, as her lawyer explained, "...a criminal trial would be a very intrusive personal experience for a complainant in this situation, given the extraordinary media attention that would be inevitable. The media coverage to date, and the efforts of the media to access our client, have been unnerving, to say the least."