...You're asking me to play my character entirely against his purpose, creation, and what he is.
He's a Sin Eater of the Advocate/Gatekeeper archtypes. He maintains the barrier between the worlds, and that means from both sides. He'll help ghosts move on if he can, but if he can't, or they're attacking people, he will show no mercy. And that means sealing or destruction. So Mike, if Rider starts attacking people, Law becomes interested because she is, for lack of a better term, a ghost.
And unless she can very, very quickly explain why she tried to murder a man just for existing, he'll seal her without question. Hell, he might just seal her even if she does explain, because she's a threat.