From the English rules' Reference Sheet (emphasis added):
When the race you are in starts, you can make a secret prophecy. You write one character’s name. If that character wins first place, then you automatically get second place
And from the rules' FAQ (emphasis added):
If the target of shadowing finishes the race, Ninja will also finish but his rank is one lower than the target because he moves after the target
"will also finish" denotes a moment where the target has finished the race and the Ninja has not; at which point you could argue that the Prophet's ability kicks in, though I think a correct interpretation is that the Prophet's ability trumps the Ninja regardless of timing rules.
Also from the rules:
In case of the “Prophet” getting the correct prophecy, the player of the “Prophet” becomes the second place winner automatically due to the special ability of Prophet