You can't really account for third party software being broken. It's outside of your control. But you should document it for several reasons:
It might happen again and your documentation will give future developers/testers some insight.
It justifies that the application is broken due to a third party integration. i.e.: it's not your fault.
It creates a ...
The short answer: you can't prevent regression in third party software.
Third party software, as one of the comments mentioned, is outside your control. Since it's essential to your software, you have a few options in how you handle this.
Regular smoke tests - Have a set of smoke tests that cover the essential functionality of the third party ...