I applied for a patent on a product/tool that I created and was actually using in the construction industry. One of my employees decided to quit and take the file and took and disappear. I recently learned that the said product was on the market and being sold. I let it go so many years ago, but seeing it on a store shelf is not acceptable!
Since the question is vague as to particulars (such as the application number), I'll try to answer generally. A patent application by itself provides no legal protection from someone implementing the invention. Only if the application gets granted as a patent do you gain protection. If you abandon the application and it publishes, it does provide some protection from someone else patenting the invention as it represents prior art.
If your former employee actually stole company documents then there may be a legal case, but not for patent infringement. In any case, you should consult with an actual attorney. I am not a lawyer and this isn't legal advice.