Options if autobody shop states part was faulty (Spark Plug)?

Car X installed new spark plugs in my 2007 Ford Explorer 10 months ago. Just recently, Explorer began to run rough. Got a diagnostic at Firestone and the result they came up with was Spark plugh in #3 was 'broken'. This should fall under the auto body shop's warrenty. I will be taking the Explorer into the autobody shop that installed the plugs last June tomorrow morning for their inspection. What are my options if they say it's a faulty part (I really don't expect them to say it was a bad installation) ?

A bad installation usually would fail immediately. If the part was faulty, a spark plug doesn't cost much, and you could install it yourself.