This is a first for me so what steps should I be taking to ensure that if my subordinate is fired there are no repercussions?
This is a classic case where, working with HR, you build a paper trail to being the process. This process should include a well documented performance improvement plan ( PIP ) that includes measurable objectives for the employee to hit. Don't be shocked if the employee actually steps up once on a PIP, and if they don't this should afford you ( legally ) some reasonable protection.
This will also vary greatly by location. As an example, in the state of GA, we are a right to work state meaning you can be let go without any reason at all given. The employer can simply say "Your services are no longer required."
That being said, this does not protect the company from a lawsuit. An effective paper trail is what protects you from a bogus lawsuit.