She might not have felt safe or able to report him when they were still a couple.
The fact he left her and moved onto you, means she has the safe space and freedom to do it now, and also to prevent it happening to you.
Also not everyone feels able to report or tell others right away, if ever.
Speaking from Experience, i was raped multiple times in very different scenarios by 3 different men.
First was in the day time when i was only 13 by a 22 year old man i didn’t know personally but in company of due to a hobby place i frequented with friends. i never told anyone ever. I wanted to years later but felt too much time had passed and no one would believe me, plus i wouldn’t know where to find him.
Second person was an ex who i was trapped in emotionally, sexually, coercive controlling and violent abusive relationship with, from the age of 18-21 and so it was frequent. I never told anyone or reported it out of fear and threats, as he stalked me for a further two years after i got free from him. I think i would have found the courage after we split if he had ended it with me like your scenario and moved on to someone new, especially to prevent it happening to another woman. I also would have reported this ex had he not been terrorising me for a further two years stalking me daily, and making my life hell. I still had heightened fear and it’s like he still had control over me long after.
Third i was I was drugged and raped on my second date, i tried getting help from the police that night before the rape took place at the point of being drugged and still locked in his house with him out, but police arsed it up, and he came back and it happened anyway, which is another story in itself, but due to this i had no faith in the police, then didn’t tell even anyone else i knew for several years again.
You work through the trauma in your own way and at your own pace. Everyone is different. No right or wrong way. Others in my position may have reported later when they felt ready. I came close many times.
I have also been sexually assaulted multiple times and sexually harassed countless times in from teenage years to 40.
I bet out of all those ‘men’ who did this are settled down with wives, girlfriends, children and having a great old life with none of their friends, family or partners any the wiser.