She may be a slag - not for anyone who does't know her to judge though (and I am in no way vindicating what has been done by Johnson - far far from it) Some 15 year olds are, as you say, impressionable. Others are not - they are manipulative and conceited and willing to do anything to get somewhere. I still reckon this was a case of "he wouldn;t pay up so I'll go to the police" (again, I'm not advocating what was done but I reckon she was hoping for a payoff and he called her bluff - bet he wishes he'd paid her now if that is the case!)
I know a number of 15 year olds that drink in places I go to and are complete slags - more sexual partners than me and I'm 40 and led a very good, play the field life before my second marriage. (Anyone remember Lorri Mattix of David Bowie fame - a 15 year old who used to sleep with famous people - obviously it was different back in the 70's but that doesn;t make it right) or the 15 year old who was chatted up by Gazza and told him how old she was - Gazza asked her Dad for permission and he was delighted and allowed him to go out with her