I’m so sorry to hear this Glanora. At 15 your DGD is old enough to decide what she wants to do about reporting this. Of course, it would be in the interests of public safety for the perpetrator to be charged and brought to justice, as he undoubtedly continues to pose a risk to other children. The police are trained and experienced in investigating abuse and working with children sensitively. She will be interviewed on video and supported throughout the process and helped to consider the implications of going to court vs. not doing so.
All you can do as a family is continue to love and support her whatever happens. It’s a truly heartbreaking situation and your anger is perfectly understandable. Please don’t alert the perpetrator as this could jeopardise any criminal proceedings, although I know how hard this is. Very best wishes to you all, particularly your DGD.