It sounds like you personally know the girl and are even possibly related to her. If that's the case, then you are in a position to provide some good support and advice. Here's a few things to consider, both in relation to reporting and not reporting (in no particular order):
1. She is in a position to possibly prevent this from happening to somebody else. I know that sounds blunt and possibly even harsh when stated up front like that, but it is a reality that all involved need to accept. With that said however, no-one should feel compelled into taking action solely for that reason. There are too many other things to consider.
2. If the matter is reported, she will have to give a very detailed statement to the Police. If the boy is charged and pleads not guilty, then she will also most likely have to give detailed evidence in a court or possibly (hopefully) via video link. This will all be very daunting and extremely difficult, especially for a 15 year old girl. It's important to remember that there is plenty of help available - but she does need to seek out that help, it won't just come knocking at the door. It's best to get this help beforehand to ensure that she is emotionally up to it and also prepared for the legal processes that will follow. A good counselor can not only provide the emotional help, but can also usually provide a wealth of information about the legal process and the emotional impact of taking action.
3. Regardless of whether or not the matter is reported (but especially if it is), it is most important that she has a strong support base around her (definitely family and possibly close friends). If it is reported, she will definitely need this, especially if she does end up having to testify. Without it, there's a good chance that she will eventually crash, especially at that age.
4. If the matter is reported, be prepared for all possible outcomes. I can't tell you how important this is. A conviction is never guaranteed, so she needs to know going in that she can cope with an acquittal if that's how it plays out. Obviously no-one wants to consider this, but you all really should because it is always a possibility. Being prepared for the worst will ensure that she can cope with it, and will make a victory all the more sweeter. Don't ever be negative - just prepared. This part is all about emotional strength - not only hers, but her support base as well. Make the decision now, that once it's over, you are all going to walk away without regrets - no matter the result. Knowing this now, makes everything much easier later on if need be.
5. If the matter is not reported, she will most likely regret it later on in life - whether it be 6 months from now or 20 years from now. Wanting to move on is admirable and in itself, shows a certain strength and resolve. But it won't make it go away. What happened, happened; and it will stay with her - it's not something you just "forget". You therefore need to consider future regret and weigh up acting now versus acting later. The longer you wait, the more diminished the chances of success.
It's all about support and her own emotional strength - and that strength will grow with the right support around her, especially from family. With the right support she will get through this regardless of whether or not any action is taken. Without it, there's a good chance that she won't.
Finally, before anything else, it is very important that she speak to a r**e counselor. Even with family and friends, it is rare that anyone fully understands the impact that something like this really has unless they've been through something similar themselves, and that can make it very difficult for the victim to fully open up and get the help that they need specific to their situation. No matter what happens from here, her chances of getting through this are greatly improved if sees a counselor as early as possible, so I would recommend that you make this your first priority.