Ask a Business Lawyer. Get Business Law Questions Answered ASAP.
It depends. Does the partnership agreement prevent such activities? I think you should have had a non compete agreement.
50 % doesn't control this issue. Was the non compete extend into post partnership formation?
The other issue in your favor even without a non compete is that the conduct here is so blatant and overreaching, it violates the duty partners have to each other NOT to engage in such acts. If I were your lawyer I would represent you; move for an immediate injunction stopping the activities; and seek money damages for stolen business.
This is information only, NOT legal advice. No attorney-client relationship has been created. Consult an attorney in your state for legal advice regarding your issue.