As koi esp. Large ones are very expensive not to mention people's attachment to them I would also leave the black spot,.
While hypo does become inert over time, I know personally that when re wetting a hypo'ed area it becomes active ( to what strength I don't know ) and foams the water and you can smell it again
You've looked at the risk, and made a decision, that's all good for you and the customer
Darran