Since the player's life total is checked only as the spell resolves, you can cast Hidetsugu's Second Rite targeting any player, regardless of life total. If the player isn't at 10 life as Hidetsugu's Second Rite begins resolving, it does nothing and finishes resolving. Note that since having 10 life is not a targeting restriction, the spell won't actually be countered for having an illegal target; it just won't do anything.