Why report it now? the police should have been notified the moment after it happened. all the evidence is now non-existent.
At this point, its a “she said—he said” situation.
Because, as mentioned by @Summum, it could stop it or at least make it less likely to happen again.
Because it could help her heal, regardless of the outcome.
Because the more people report rapes at any time, the more likely it is that people will feel safe reporting rapes.
Because this may be the first time that she’s been able to admit it to anyone, even herself.
As to the OP – it should always be reported when it happens. However, for some this might not be physically possible for some reason, or emotionally possible for many reasons. The problem with reporting it later is, as mentioned, the loss of evidence. Therefore, you need to be sure what your friend’s reasons for wanting to report the rape are – because if she’s looking for justice, then the courts are probably the wrong place to go.