Good for her.
It's unlikely there'll be a prosecution, as the police/system is crap about rape. So she can be fairly safe that she'll 'just' be putting it on file in case anyone else reports issues with it in future.
In the very, very, very rare event it comes to a prosecution being about to take place and she has been told that for sure, she can always drop it at that stage. It might even still go on file then.
Yes, in theory the police could bring it forward without her but that wouldbe even more rare I think, especially for rape by a then partner (because, as I say, the police/CPS are shit.
What I will say is she shouldn't expect anything, and she shouldn't expect to be treated particularly nicely at all times, their amount of kindness varies depending on what they've decided etc. If she goes into i without expecting anything, the process is less likely to scar her. Go into it knowing that the system is misogynist etc at every step along the line, and she will have to know her own truth inwardly and not let any unpleasant comments/treatment she receives effect her.
I am reporting abuse etc that began 18 months ago at the moment- saw an officer this week. I'm trying to maintain no attachment at all to the outcome as much as I can. I'm just putting this forward as how his behaviour is, and how they consider it/what they do with it is up to them. I'm just hoping it'll lie on file.
Best wishes and well done to her from me. xxx