I currently support several applications that use web services provided by an external company.
One of the key frustrations is when the company radically changes one or more of it's services .
This breaks our applications and documentation from the service company to support updates is patchy .
This happens periodically .
Sounds like a frustrating situation and a difficult one to address when dealing with an external organization.
Ideally, the external company would use the Model Accelerator, and in turn models, as a foundation for their documentation. I'm a fan of using models (and in turn diagrams) as a means to communicate. Having diagrams to share before implementation would also provide an opportunity to share documentation with partners in advance of development (and deployment), giving partners a chance to provide feedback and prepare for the change. In addition, using approaches such as goal-service modeling and service litmus test would hopefully help them to improve alignment with their business needs and solution quality - leading to less overall churn.
In essence this would help in boosting their design time governance efforts, but it sounds like there are a few issues with their run-time governance as well. These issues go beyond our areas of focus, but some things to look at would include working toward SLAs, including details on frequency of change, documentation required, opportunities for feedback/input, migration procedures (including keeping old and new services running during transition).