I'd explain why the problem occurred and explain it's incredibly normal for fonts to have some letters/numbers bigger than others. I might even take 3 minutes to type in a few fonts and show how common it is. I'd also go ahead and type in his digits all sized to 6" and show him it will probably look stupid as the weight of the digit will now be off.
I'd then want to know what's the problem with the one digit being an insignificant amount taller. Will he not pass inspection? Will it not fit in the space he needs to put it? What's the issue?
Typically after you educate someone and lay a bit of logic on them, they'll come around. If it's still an issue, fix the problem, and move on with your life. If he comes in again, you'll have his file setup so it should make it pretty painless to do it again. If he wants something else done, just tell him you're too busy or make up some other excuse.
I'd then want to know what's the problem with the one digit being an insignificant amount taller. Will he not pass inspection? Will it not fit in the space he needs to put it? What's the issue?
Typically after you educate someone and lay a bit of logic on them, they'll come around. If it's still an issue, fix the problem, and move on with your life. If he comes in again, you'll have his file setup so it should make it pretty painless to do it again. If he wants something else done, just tell him you're too busy or make up some other excuse.