One - does common English usage equate that to "you" (2nd person) or "he/she" (3rd person)?
Solution 1:
Traditionally, it should be one's here. If you introduce one, it should always be one's, oneself, etc. when referring to the same impersonal entity.
Burchfield tells us in Fowler's Modern English Usage that his was sometimes used in America, but he agrees that it is now conventionally one's everywhere.
Solution 2:
If you use one as your pronoun, you should not substitute in either you or s/he to represent the same person.
The prose should be consistent throughout:
One must mind one's manners.
You must mind your manners.
He must mind his manners.
etc.