I don't understand this - if there is clear evidence available that a serious assault occurred, how is it up to the victim to decide what the punishment is? Can't police prosecute the guy independently, and if so why wouldn't they?
Women seeking justice for sexual assault face daunting obstacles; here's one example:http://freethoughtblogs.com/pharyngula/2013/12/10/whats-it-like-to-be-a-woman-victim-of-assault/
Please note I'm not saying the police who handled Ms. Hessler's case were as unprofessional as the ones in the incident I linked to. But there are a lot of reasons women may choose to not report, much less press charges, about being sexually assaulted. They might be afraid of further violence against them, of being stigmatized, of censure from others (even friends and family). They might even be ashamed.
And police, at least here in the US, often trivialize such crime. They and other law enforcement display a marked reluctance to follow such things up unless pressure is exerted. There's a well-established culture of victim-blaming with regards to violence against women.
All this may have some bearing on Ms. Hessler's particular case. The article doesn't go into detail.