I had a run in with a mentaly ill 14 year old when i was a teenager.I met her once at my front door so she could collect something from her cousin who was my brothers gf at the time.She never entered my house and all i did was say hi and gave her what she wanted(some backpack). My bro and his gf got home and were looking at me weird saying that she told them i kissed her and that im her bf now,i was like WTF!!?!??!?!?!
Luckily her cousin knew she had issues and proceeded to tell us that she had ran away from home before multiple times,cut herself and accused people of rape for no reason. She even lied to my cousin when he went out with her for a short time and said she was preggo even though they'd always used a condom and she was on the pill
I wouldnt get involved if your not 100% sure,just give her sympathetic support when needed and try to get her to tell the police and explain that if he did this then another kids life and mental health is at stake,if its all true and she goes to the police then justice will run its course(hopefully) if not then she's just gonna avoid it and theres nothing you can do anyway.