Loosing clients is just a fact of life no matter how good (or bad) you are. Be it bookkeeper or accountant. Professional courtesy should remain paramount for all parties but in the real world sometimes it doesn't happen. Just remain professional and to the point on your side at least.
We've all lost them but more often than not, another one takes over soon afterwards. It should not be the end of the world unless you are one of these 'all eggs in one basket' types with just a few very large clients. Possible lesson learned on practice fee structure?