They are a statue of the beast form.
Petrification would not change their form, except to turn them into a stone statue of whatever beast form they were in. There is simply nothing in the definition of Petrified or the description of Wild Shape to indicate otherwise.
When reduced to 0 hit points, their statue appearance does not change, but you substitute back in the druid’s usual stats.
The Petrified condition states:
A petrified creature is transformed, along with any nonmagical object it is wearing or carrying, into a solid inanimate substance (usually stone)
So the material of the now petrified beast form is solid and inanimate. However, conditions propogate through WIld Shape transformations. So when reduced to 0 hit points, they are still petrified.
Now, when reduced to 0 hit points, Wild Shape states:
You automatically revert if you fall unconscious, drop to 0 hit points, or die.
So you revert back to being your usual self, as in, you are back to using your normal druid stats. But because your physical form has been petrified and is now solid and inanimate, the form of the stone would not change into your usual druid form until the petrification ends.
To give a needed caveat, this is, I think, the best reading of the rules for this interaction, but there is room to rule that the appearance of the statue is now that of the druid. And to be clear, the only difference between these two rulings appearance. Regardless of how you rule the appearance of the statue, the statue goes back to using the druid’s usual stats when Wild Shape ends.