Hi Mucj,
If the accountant was acting as your agent or representative, then they may owe a fiduciary duty to act in your best interests. If this is the case, you might have an action at law. However, I suspect they were just providing accounting services to you and therefore, was probably not acting as your agent or representative. In this case, there is nothing under law (that I know of) that prevents them from acting for both sides. However, is the accountant part of a professional body? If so, there will probably be something in that body's practising ethics that recommends against acting in such circumstances unless the accountant gets approval from both parties.
Did the accountant tell you they were also acting for the other side? Was the accountant found by you or was it recommended to you by the other side (e.g. to save costs)? Did you get your business terms checked by an independent lawyer?