Can Business 1 sue Business 2 that was started by an employee who worked at Business 1 while opening Business 2?

An employee at our business spoke about opening a business of his own that was supposedly over 90 miles away. As time went on, a clinic was being constructed down the street from our cljnic. We found out it was his and he was still attempting to deny that it was his but finally admitted that it was his after the owner himself told him he had to leave. The employee asked why and the owners response was that it was a conflict of interest. The employee had knowledge of our prices, access to our clients, and access to our equipment/vendors, etc. Since his business opened, our business lost many clients and we are losing a lot of profit. This isn't fair and we were wondering if what he did was legal and if he can be sued.