What would you do if you unknowingly slept with a long lost father/child?

I watched this movie last night where this guy, as a revenge plan, went out of his way to hold a man hostage for 20 years (he had a 3 year old daughter so by the time he got out she was 23 and had no idea about her biological father nor did her father know what she looked like). 
Anyway, after he was released, he happened to bump into this girl and they had a sexual relationship, only to find out that the girl was his daughter... He cut off all contact and had 20milion worth of diamonds delivered to her with a note explaining how shes very special and how he will always love her but they'll never see eachother again. He didn't tell her that he was her father as that would obviously mess her up more than she already was. 
Would you tell them? Or would you leave?

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