Late to your thread but thought my experience might be helpful. It's not the same but I made a historic CSA allegation comparatively recently to the police. There was a lack of forensic evidence obviously but I thought there might be some corroborating testimony from others. Either way, it was important to me to make the report. I made the initial report at a local police station and was then interviewed by telephone at length and in detail by an officer who wasn't specifically trained and made a few (imo) off-colour remarks. Because the offences had taken place in a different area, I was then passed to a different constabulary and went through the same process with a different officer who was much better, again by phone. He arranged to do an in-person video interview with me, but before that could take place, my case was passed to a different unit at a different station and I was contacted again by a new officer, this time a trainee detective, who was utterly inept and unprofessional.
I did the video interview about 6 months after my initial report. My testimony was solid, detailed and consistent, but what I didn't realise in advance is that when the police take a statement like this they make you go over and over it, breaking it into its constituent parts and then going over each section again, over and over in a more telescoped way. I understand why they have to do this, but I wasn't warned in advance, and didn't know that this is almost the exact opposite of how a therapist will encourage you to talk about traumatic events. It probably also didn't help that the detective was so inexperienced. To cut a long story short, I ended up with PTSD, and adding insult to injury the case was dropped in fairly short order, supposedly for lack of evidence, but mainly because the police didn't really bother to pursue any. (I was informed of this by email on Christmas eve. The detective ended: "Have a lovely Christmas x". With a kiss. For real.)
I realise that a lack of resources means manpower has to be rationed, but if I had realised what that would look like in reality, and if I had understood more about the interrogative process, then I would have balanced those two factors against each other differently, and might well have made a different decision, even though making a police report felt extremely important at the time. I certainly felt, and still feel, that if I was raped now, I would proceed with great caution, having felt previously that I would 100% report and have my day in court. If I had a 15-year-old daughter, these are the things I would keep in mind. I don't think there is one right course of action, but if you - or she - are hoping for a thorough, skilled and sensitive investigation with the morally right outcome, you may well be devastated by what you get instead. Helping her hold it together for the sake of her education only for her to fall apart at some later date because of the unfinished business may well be a better path imo. There will be fallout either way. I'm so sorry this has happened.