I'd need to know how old she was, and what was involved in the "fling".
Was it exploitative or abusive?
Did he shame her in any way on social media etc.?
How did this fling end? (broken heart, regret ... )
There's too much missing from the story to know for sure.
Anything that would make me confront someone in their workplace would also be suitable for me to call the cops.
I never did that with my daughter's longterm bf even though he was a dick and basically kidnapped her.
I never did it for my son even though one gf chased him through my house with a hammer.
*Correction -
I did call the cops about the kidnapping, but not against him specifically.
It was when she was still missing and I had no idea where she was.
They refused to help because she was almost 18.
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And in the end, the love you take is equal to the love you make.