First and foremost, NEVER lie on an immigration form. They can take that as committing fraud and can lead to a denial and deportation proceedings.

Now that that's out of the way... If they're being petitioned by a USC spouse or other direct relative then any unauthorized employment is forgiven.
Either way though, if they filed taxes then USCIS knows they've worked unauthorized so best to just be honest and put yes. You will get to explain why you worked on the last page of the form. Most people who know it will be forgiven write something along the lines of needed to work in order to provide for my families needs etc