You should use the same pronoun throughout a piece of writing. If you use you in a sentence, you must use you as the pronoun, and not switch to another.
While you are gardening with your mother, one should work as hard as they can.
This sentence first used the pronoun you.
Thus, to keep the pronouns consistent, the sentence must be:
While you are gardening with your mother, you should work as hard as you can.
*This is also a leading mistake in the essay portion of the SAT Writing section.