Not every marriage is a success. Sometimes you discover that your're better off going seperate ways. If that happens to you, end the relationship in the same professional way that you began it. Talk to your client. Explain what's going on. Chances are it won't come as a complete surprise.
You may even be able to work things out. But even if you can't, it's better to acknowledge what's happening and bring closure to the relationship than to just walk away and disappear