First, I would try to verify that diagnosis yourself (or take it somewhere else to have verified). It would take a lot of abuse to round off the body of a plug and it's my guess that damaging the the threads in the aluminum head would occur first.

Second, if this were truly the case, you can purchase (or locate a shop that has) what's called a bolt extractor. They are essentially a socket, and you want be sure to request a deep socket. Example of a kit: Sears: Online department store featuring appliances, tools, fitness equipment and more