Hypothetically speaking, scam a scammer? Legal/illegal?

Hello, I have a penpal and I have already reported this case to proper authorities in the 2 specific countries so this is for my knowledge...

I known her forc10 years, she is from a under developed part of a country known for mass scamming, but she is sweet and doesn't scam or never did until now... her dad legitamently fell ill, I found iut through her uncle I know here in town, she didn't want to tell me, so she tried to work abroad to pay for treatment, she came across a man she was 100% taken with and offered to send her down essentially 18k usd for her hand in marriage, 8k is how much her dad's surgery will be, he shown all signs of a trafficker catfish, I was drilling questions specific to jt, and so I sent a documented email with everything to proper authorities, well when I confronted her, explained it, she seemed terified he has her address etc etc, then she decided to scam him since he is trying to scam her, I talked her out of it but unfortunately they known eschother 8 weeks next month and will be getting married, as she can't back out, she is scared for her life knowing this, but the police will be at the airport to help her either way...

She can't tell him she knows, would put her family at risk, so he is sending down 3 payments they may see 2 payments, and 2 payments would fix her dad but she isn't trying to scam she just can't tell him, would she get in trouble or have to return the money used for her dad's surgery? Even know it was from a kidnapper marriage and now she doesn't want it but would help her dad? Jw