He got some more info from speaking directly to the girl finally, and it turns out he very likely is the dad. He literally knew her mom for 3 hours on a bus ride to Reno, both of them hammered out of their minds, when he was on a mini-break from his future wife when they were fighting, and banged her in a car when he drove her home from the bus station, which is why he didn't remember it until he heard the story. The mom told the daughter that she never looked for him because there was nothing between them except a drunken fling and he didn't owe her anything. Which is great, except that the girl wanted to find her dad of course, and now his wife and daughter can't understand that their lives are vastly better for him being in their lives, no matter what mistakes he made early on. His son completely understands at least, and basically said all the same things I have been telling him, but we'll see what the cunts end up thinking after they've cooled down.
I so much wanted it to not be him, just so he could hold it over their heads forever for being such judgy cunts. Ah well. If there's anyone still young on these boards, remember to wrap that rascal! You never know when it might come back to haunt you.