That's a toughie, especially since Soon's a man we don't know much about (unless there's more to read about him in OtOoPCs or SoD, neither of which I've gotten yet). He seems like a pretty die-hard Paladin though, so I'd say that first he'd put all his energy into talking him/her into Atonement, then he may try to Atone him/her using force (or he may just do that first; what's 500 xp to someone of his level anyway, especially since this his own flesh and blood we're talking about?). If all that failed, he'd either Smite away or... take the "honorable" alternative to Falling.
Just like Tsukiko!
My goodness. Sounds like you've been having some rather fascninating conversations.