Adapt to the knowledge you now have. For whatever reason, your coworker divulges information in ways that exploit you, or "throw you under the bus." From here on out, your mission is a preventative one. Value your reputation very highly, and never give your coworker ammunition against you. You will be tempted to confront your coworker about the problem. If you must, you must, but I would advise against it. Complaints will be complained about in turn, as your coworker has clearly proven.