Spark Plug Blow Out

I'm posting about my Mom's 2003 Subaru Outback. It has 70k miles and is not under any warranty. The passenger front spark plug blew out of the cylinder head and the plug would not thread back in. Have any of you experienced this and how difficult will it be to repair? I'm praying it will not need a cylinder head. Please help!

You may be able to rethread the spark plug hole, or have someone do it for you. My bigger concern is why the plug "blew out"...

The previous owner had installed spark plugs and misthreaded that plug. The threads were trashed. The local subaru shop <not a stealership> had to remove the valve cover and timing cover to install an insert or helicoil. Total repair was just over $400 parts and labor. That included 4 new spark plugs and wires. Car runs like a champ now.