This looks to be a vendor specific question about the Brilliant Accounting product and its capabilities. You should get in touch with them and find out if their client/server model is routable and how much bandwidth each client will need. Multiply that bandwidth by the number of clients and find a VPN solution that will accommodate. If this isn’t possible, you’ll be looking at a telco provided solution between buildings.
Other option, with specialized equipment and line of sight between the buildings, Point to point wireless may work.