1) A simple answer would be this a case of fiduciary relationship - where she was under the power and influence of someone she viewed as an authority and complied because she assumed she had no choice. Thus it would be considered rape and not seduction. However while that is clearly the halacha for a child - it is not the halacha for a 12 year old who is considered an adult. However I can not find any teshuva where fiduciary relationships are a mitigating factor. In fact even in secular law there is no consistency. A rabbi is not considered a special authority figure in many places - as was seen in the case of Mordechai Tendler.
2) He can be reported to save him from sin. It can also be justified because he is a rodef for prohibited sexual relations with a nidda - even if consensual. And thus the police can be called as a way of saving him from sin.
Rambam (Hilchos Chovel u’Mazik 8:11), And thus all those who disturb and distress the society it is permitted to hand them over to the secular government to be beaten or imprisoned or fined.
Thus someone who is a public nuisance can be turned over to the police - even if he will be imprisoned. The Chasam Sofer, Minchas Yitzchok (8:148): and others note this is connected to Gittin 7a and that even a private nuisance can be turned over to the police if there is no other way of stopping the abuse.
Chasam Sofer (Gittin 7a): Mar Ukva said that there are people who are irritating me [verbal – Rashi]. … Even though it was only verbal abuse, nevertheless if it wasn’t for the fact that Mar Ukva could save himself from this abuse by arising early and going to the study hall - it would seem that he would have been allowed to report his abusers to the government… We see from Rashi’s explanation that it was clear that if Mar Ukva had been abused monetarily or by forgery he would have been permitted to report his abusers to the government and he would not have been required to go to the study hall. That is simply because if a person comes to kill you than you have every right to kill them first. This is also the ruling of the Rambam (Hilchos Chovel u’Mazik 9:11): “And similarly if a person abuses and harasses the community it is permitted to give him over to the secular government to be beaten… In contrast if only an individual is being harassed it is prohibited to report him…” This would indicate that the Rambam is referring to verbal harassment. Therefore if the assailant caused an actual loss it would be permitted to report him to the government so that he doesn’t cause further loss. This is the ruling of the Rema (C.M. 388:9) and it is also the ruling of the Shach (C.M. 388:59-60). However while it seems obvious to the Rambam that if one verbal harasses a community it is permitted to report him to the government – the commentaries don’t show the sources of this ruling. It must be that this ruling is learned from this gemora. It would seem that if hadn’t been for the fact that Mar Ukva had a solution to the harassment problem by going to the study hall it would have been permitted to report his assailants to the secular government. However this solution is only relevant for an individual. But it is not relevant for the community and therefore it is clearly permitted to report the community nuisance to the government. Also see the Pnei Yehoshua’s discussion of this gemora where he says that the solution of going to the study hall and complaining to G‑d about the assailant is not correct if one can stop the harasser in other ways. However he says if the harassers cause him to waste time from Torah and prayer because of his upset he can stop the harassers in any manner…
Protecting others from a sexual predator obviously is a public nuisance as well as a private one. A similar problem is discussed in Rav Sternbuch:Calling police for Jewish owned house of prostitution.
Consequently it is not only permitted but it is a mitzva to report the existence of a predator to the police or a therapist/teacher who is a mandated reporter who in turn will call the police - even if it results in the predator going to jail or losing money.
In sum, it is clear that even though a 12 year old is an adult according to Jewish law - but she can and should call the police to stop the predator from committing sin as well as to protect her well being and that of others. She also can report the matter to a mandated reporter such as a therapist or teacher- who will report the matter to the police. Reporting a predator is a major contribution to the well being of society and is to be strongly praised.