The Inmate. Freida by McFadden

Brooke Sullivan, the new nurse at a maximum security prison, has only three rules to follow: treat the inmates with respect, never reveal personal information, and never get close to the inmates.
But what if Brooke has already broken those rules? No one on the staff knows that one of the most dangerous inmates in these walls is an acquaintance of hers. A very close one, even. And no one knows that in high school Brooke had a crush on Shane, a star quarterback who is now spending the rest of his life in prison for a series of horrific murders. And he ended up here because of Brooke's testimony.
But Shane knows.
And he will never forget it.